1"IS7iF?P.7FïS !1?^l) i^`^1R^ii^±`. SERVICE
'Ir-an>,IatIon Series i,,t^, 4339
Studies of gü.St:rlllalï.Loi1 in vertebrate meroblasts
I 7.'c'l.e.osi.s
by J. Pasteels
tit:Ie:
Etudes sur
f la gastrulation des 'V(?ï't:(?br; û IlI^E'Yob1.8ût1.ClLlcs
I. 'rë1.i.ost6r:n:.^
O rom:
Arch, F3:i.o1.. ALV:f_I: 2()6-303, 1936
JI-n]7s1.8tî14I
by
i:.hE.
i..l'F!Y15lct;.7:)T! :,E9Ct:2Oi1
r)C:.l>al"(:Illr1?L
Of the
T?,2j731'titlCïlt-
of f=11 =: FriV:i.I"OI?ï; E.Df
Service
l'_isili^_'I:i.es %1_If^1 l'i'<I? i.]'l(
F^é1C:i f1.C B:lo1.C)g1Cal Station
1\aili?lIlli),
102 Pagc_r L^ hesCr 7-p c
B.C.
ARCHIVAS
de
BIOLOGIE
•
founded by
Ed Van Beneden and Ch Van Bambeke
continued by
0 Van der Stricht and A Brachet
then by A Brachet and à de Winiwarter
Published by
H de Winiwarter
Professor at the
and
P Gérard.
-
Professor at the
University of Liége
University of Brussels
with the supportof the
Fondation Universitaire de Belgique
•
Volume XLVII
Lig e
Paris
11 Vaillant-Carmanae (SA)
Masson et Cie
Imp de L'Académie
Edit-libraires de l'Académie de Médecine
4, place St-Michel
120, boulevard St- Germain
1936
Studies of Gastrulation
•
in Vertebrate Meroblasts
Teleosts
by
Pasteels
(Laboratory of Embryology, Faculty of Medicine, University of Brussels)
(37 figures in text)
TABLE OF CONTENTS
Introduction
•
(1) Background
(2) Materials and method
II Personal Observations
(1)
General outline of developmenton the basis of macroscopic
observation
•
(2)
The blastula - formation of the subgerminal cavity
(3)
Distribution of organ-forming territories on the surface of the
blastodisk at the beginning of gastrulation
(a) Organization of anlagen in the blastula
•
(b) Details of the experiments
(c) Comparison with other foras
(4) Morphogenetic Movements
(a) invagination, prostomial thickening, origin of the endoblast
(b) Convergence
(c) Extension
(d) Structure and significance of the terminal hc..ede
(e) Epiboly and divergence
(f) Comparison with other forms
Archenteron and roof of the archenteron, endoblast and hypochord
(5)
Conclusions
(1) Gastrulation in the trout
(2) Extension to other tcleosts
•
(3) General
Summary
__Bibliography
. 206
INTRODUCTION
1 - Background
The experimental method, which has replaced the often sterile
process of phylogenetic "explanatioe has made possible remarkable
development of the science of embryoiogy in recent years and at the
same time, far from bringing discredit to the major themes of comparative anatomy, has renewed interest in them. The search for homology
has become all the more fruitful since it is no longer based solely
on the comparison of forms that develop at the various stages of
ontogenesis but as well on the study of morphogenetic factors and
mechanisms.
•
•
In this connection, gastrulation, the tremendously important
step in which the developing animal changes from a formless blastula
to an embryo with organs, domands particular attention. Purely
descriptive embryology, based only on the study of serial sections,
has not been sufficient to determine precisely what the mechanisms of
gastrulation in vertebrates are. This phase of morphogenesis is too
dynamic to be followed by an essentially static method. Only continuous
observation of the living egg can provide accurate, definite data.
Kopsch attempted this and in 1895, was able to determine the direction
of cell movement during gastrulation. in Axolotl eggs by careful
examination of natural pigmented markings. But it is Vogt (1925, 1929)
to whom we are indebted for a new experimental method. He was the first
to prospect the territory of the living egg using localized coloured
markers. His technique, though it appears bold on first consideration,
proved fruitfulindeed. A piece of agar thoroughly impregnated with
vital stain (Nile blue, neutral red. or Bismarck brown) was applied to
the surface of the egg so that it stained a well defined group of
cells. The stain did not harm the organism, did not diffuse beyond
the area of application and persisted well enough that the movement
207
and eventual fate of the marked group of cells could be observed.
By marking the surfaces of amphibianhlastulae (Anurae and Urodelae)
in this way, Vogt (1925, 1929) was able to achieve two things: (a)
he mapped out exactly the areas on the surface of the blastula
that correspond to the major aniagen of the embryo and (b) he found
out what movements the anlagen had to undergo in order to become
organs. The extent of the rearrangement had been quite unsuspected.
These collective movements of cells that occur during gastrulation
can be called "morphogenetic movements" (Vogt's "Gestaltungsbewegungen
The coloured marker method had given such good results that it
was soon. applied to other orders of vertebrates. In 1929, Graper
filmed blastodisks stained in toto
and locally stained blastodiks,
_.
giving us our first idea of the morphogenetic movements of the
primitive streak in the chicken. Also in 1929, Wetzel published
data obtained for the same species through systematic staining of
a number of blastodisks at various stages in the first two days of
incubation. Although the theoretical interpretations of the two
authors differed in some respects their main observations were in
agreement . In so far as fish are concerned, we owe to Weissenberg
(1933, 1934) several publications on gastrulation in the lamprey,
which he studied by the same vital staining methodl. In the meantime,
Conklin (1930) resumed the study of development in Amphioxus and
through meticulous tracing of cell-lineage, was able to rectify
the errors of earlier workers.
•
1
Kopsch had employed the vital staining method in Scyllium as early
as 1926, but the embryos he used in his experiments -were too old
for his work to be decisive. Not long after I began the present
work and published preliminary results (1933,1934) Van De. Broek
(1935)
did a similar study on. gastrulation in Scyllium
—
•
208
We now know that the mode of gastrulation in the Amphioxus egg
is the same as in ascidians.
The morphogenetic movements of the eggs of amphibians, birds,
cyclostomes and prochordates follow the same general pattern, -as we
shall see. Will those of the groups not yet studied - meroblastic
fish, reptiles and mammals-turn out to be similar? To what extent
are the movements modified by the yolk? In what way is gastrulation
adapted to accommodate a huge food reserve? These problems are by
no means the only once in the field that merit further research. There
is no need to stress the importance of knowing exactly how
gastrulation occurs in mammals; for fish there is the question
whether the embryonic trunk forms by convergence or concresence;
in reptiles there may be transition between typical gastrulation
and the primitive streak . These are theoretical questions of
Immense importance.
•
Let us summarize the data already obtained for amphibians,
birds, cyclostomes and prochordates. (For information on the latter
one may refer to Conklin's work on ascidians and Dalcq's more recent
work on merogeny in the unfertilized egg.) In ail these groups
the arrangement of the organ-forming fields on the surface of the
blastula before gastrulation shows great similarities. The anlagen
are established in the horizontal plane, forming crescents more or
less at right angles to the animal-vegetal pole. Vogt 's description
of anlagen in amphibians (1929) is so well known that we shall not
dwell on it, but mention . only that the chordnresobiàqt forms
a complete marginal ring. The notochordal area is dorsal and is capped
by a crescent destined to become neural tissue.
In cyclostomes the arrangement is broadly similar (Weissenberg,
1933, 1934). In these animals, however, the organ-forming regions
are concentrated dorsally. The marginal zone is discontinuous on the
•
209
ventral aspect, which is purely ecto- orendoblastic. As in Anurae
and contrary to the case in Urodelae, the substance of the lateral
mesoblastdoes not lie on the surface but is hidden under the
endoblastic zone of the archenteron. In selacians we find a similar
pattern (Van De Brock, 1935), Like the lamprey, they display a
wide ventral interruption of the marginal mesoblas tic zone.
Wetzel's map of anlagen on the surface of the unincubated
chicken blastodisk (1931) is sketchier since it could not be
established directly. The arrangement is in general the same as in
amphibians; however, the author attaches great importance to the
"ruhend indifferent" material of the primitive streak, which be
feels to be similar in composition to a tail bud.
ascidians and Amphioxus - (Conklin, 1905,
In the prochordates
mesodermal
and notochordal regions are clearly
1932; Dalcq, 1932) the
dissociated at the level of the marginal zone. While the prospective
chord is in a crescent directly underlying the dorsal neural crescent,
the mesoblast forms a median, but ventral, crescent.
•
•
In. short, despite undeniable differences, there is clearly
great similarity in the arrangement of the organ-forming territories
of all the chordates studied until now. If one wished to define their
principal characteristic, one might say that the anlagen are spread
out horizontally. In the body of the developing embryo these cell
groups grow lengthwise. Of the main morphogenetic movements, two in
particular stand out: a movement of axial concentration, or convergence,
and a movement of elongation, or extension. Convergence and extensio n.
are combined with the invagination typical of gastrulation. At the
same time as the marginal zone actively invaginates to form the roof
of the archenteron and the vitelline mass is brought inward to
210
•
•
•
constitute 1 the lateral walls and floor of the archenteron, intense
epiboly in the ectoblastic zone results in the ectoblast covering
the entire embryo. These morphogenetic movements were described and
defined by Vogt (1929) in his masterly study of gastrulation in the
amphibian. Previously they had only been suspected. Often, what
workers in the field had not dared to attribute to cell movement,
they attributed to cell growth and proliferation; when it was
recognized that movements occurred, they were wrongly interpreted.
The movement of axial concentration could not be missed, but most
authors (Hertwig, Roux, Brachet, Kopsch, Kingsbury), including those
who wrote the major works on embryology (llertwig, Brachet) thought 4t
to be concrescence. Concrescence is opposite to convergence in the
sense that the former term implies a joining together, a fusion in
the midline of two equivalent halves separated at an earlier stage,
whereas by convergence is meant that as soon as development begins,
the prospective tissue is a single entity, containing median material
in its central part. As for the movement of invagination, it was
rarely recognized as a movement. Most authors interpreted the
formation of deep layers as the result of intense cellular proliferation
in the area of the blastopore. Others, such as Brachet (1902) felt
that the layer was laid down by a process of delamination, of
"gastrulean cleavage".
We have seen that there is great resemblance in the arrangement
of anlagen in amphibians, cyclostomes, birds and selacians, and
their morphogenetic movements are also similar. The saine five
components - invagination, convergence, extension, epiboly, and
ventral divergence'- are - found in all, but in diverse combinations.
The process of gastrulation in Petromyzon resembles very much that
in the amphibian, but it occurs onlY in the dorsal part of the
blastula. In birds, it seems that the formation of the primitive
streak results from actual chronological dissociation. The observations
211
of Cr'dper and Wetzel give one to understand that invagination is
preceded by movements of convergence which bring material into
the centre of the blastodisk. Invagination then occurs not by
reflection of the layer but by infolding of cells along the
primitive streak. Extension, according to Wetzel's data, does
not take place until the primitive knot recedes. However, there
are some obscure points and two recent studies (Kopsch, 1934;
Twiesselmann, 1935) contradict the results of Graper and Wetzel.
In view of this, gastrulation in birds merits further study.
With the concept of morphogenetic movements came an end to
conflicts of theory that the exclusive use of the microscope and
the pen had threatened to prolong forever. The notions of
appositional growth, of the deutenteron and the enterocoel
have been found to be invalid for all forms examined until now.
As we have seen, the concept of convergence has replaced that of
concrescence. Hi s developed the concept of concrescence when he
studied gastrulation in teleosts (1873, 1876, 1978). The extension
of the idea to amphibians, cyclostomes and to Amphioxus has, of
course, proved to be incorrect. But could it nevertheless be truc
for fish? Could the effect of the vitelline mass not be to cleave
the aniagen, each originally single? As Vogt remarked, "In
meroblastic fish lies the last bulwark of the concrescence theory
and at the same time the first point of departure from it," (1929,
p 673).
•
•
To keep to teleosts, which Shall be exclusive subject of this
first article, the'concrescence theory has not persisted without
modification since the time of His. According to the latter author,
the envloping border situated on either side of the posterior
extremity of the embryo folds back behind the embryo, fuses with
the median line and prolongs it by integral concrescence. It was,
however, recognized that growth of the "terminal node" itself (the
•
•
•
dilated posterior extremity of the embryo-forming area) could not
be ruled out.
After a thorough study of embryonic development performed on
living Ctenolabrus eggs, Morgan took vigorous exception to the
ideas of His, stating that the only process occurring was one of
axial development. In the same study, Morgan showed that after
excision of the enveloping border the embryo continued to grow. The
fundamental importance of the terminal nod e w as once again brought
out by Sumner's experiments with the implantation of slivers of
glass in various teleosts and Kopsch's electrolytic destruction
of parts of trout embryos (1904). Kopsch concluded that at the
beginning of development there is a cephalic "growth centre" in
the middle of the posterior extremity of the blastodisk, with "body
growth centres" on either side. The terminalno de (the protruberance
in the middle of the posterior edge of the embryo, Figure 1C and
D) formed, Kopsch suggested, as a result of concrescence of the
two body centres behind the cephalic centre. The posterior no de
have only to grow in length, while incorporating the wouldthen
material from the enveloping border in the lateral portions of the
embryo. This was the form in which the concept of concrescence in
teleosts appeared in the major texts of embryology. A few authors,
howeverdid criticize the Concepts of Kopsch. Moser (1907) observed,
with good reason, that the localization of lesions through the opaque
chorion must necessarily be less than precise; moreover, among
Kopsch's embryos there was neither a typical hemiembryo nor one
with an exclusively cephalic lesion and an intact trunk. These were
the only data that could have provided the author with truly
decisive arguments to back his case„ Moser added that Kopsch's
results could very well be interpreted as indicating that there is
anteroposteriôr head-trunk localization right from the beginning
213
of development; the same thing would apply to the partial twin
embryos that are occasionally encountered. To these excellent
criticisms can be added another of a methodological nature: we
cannot hope for any true progress in our knowledge of morphogenesis
as long as we use concepts as ill-defined and equivocal as that
of "growth centres". This is the main fault that can be found with
some of the concepts echoed by Veit (1923) in the most recent study
of morphogenesis in fish. Gastrulation does consist in the formation
of layers from which the primitive organs subsequently develop.
This is the principal concept of the classical embryologists. It
is only the imperfection of their means of investigatio n that has
given rise to inconsistencies.
•
After preliminary studies by Lereboullet (1854, 1861) and Kupffer
(1868) Goette, in 1873, published the first comprehensive study of
gastrulation in the trout. According to Goette, the deep "endodermal" 2layer originates, as in other vertebrates, by true invagination, and
thus by reflection
of the superficial layer. In the same year (1873),
however, Oellacher proposed an entirely different thesis - that
the endoderm developed through cleavage. This idea was upheld by
Ris (1878), Hoffmann (1881), and Ryder (1884). In a later study,
Kupffer (1884) tried to demonstrate that the median groove ("nckenfurche")
found at certain stages in the young embryo is equivalent to the
primitive streak of amniotes. Hennegly (1888) hdd no aifficulty
arguing against thisopinton, in his study of the development of the
trout from segmentation to the formation of aniagen. This very thorough
Y•e•
1
•
•
By "endoderm", the early authors meant both the deep layers, which
we now consider to consist of the chordamesoblast and the endoblast.
The present author is using the terminology proposed by Calcq and
Grard in the new edition of Brachet's Trait8 d'Embryolorrie.
214
•
study marked a decisive stage in •our knowledge of teleostean
development. Henneguy felt that the "endodere formed by a process
of invagination in which cellular proliferation in the iii s of the
blastopore would play a greater part than cell movement. According
to Wilson, invagination in Ctenolabrus exhibited the additional
feature of delamination from the centre to the periphery. Samassa
(1896) argued vigorously against the thesis of invagination, which
had found another proponent in Kowalewski (1886), and suggested that
there was delamination complicated by appositional growth on the
median line. Sumner (1904), in the noteworthy description accompanying his experimen7:
work, returned to the thesis of invagination, saying that it
occurred at various levels for the chordamesoblast and the endoblast.
Boeke (1903) felt that there was a combination of invagination and
delamination in.buraena. But three years later (1907) the same
author, influenced by the work of Brachet (1902) on the amphibian
egg, was of the opinion that there was "gastrulean cleavage", forming
a potential blastopore which subsequently became a true one; the
envelopment of the yolk was a process of notogcnesis only, More
recently Chevey (1925), in a study of the living, transparent perch
egg, has returned to the thesis of invagination. A second study by
Goette (1878) and a paper by Goronowitsch (1885) considered the
formation of the medullary plate. The origin of the endoblast was
studied by Kowalewski (1888), Boeke (1903), Summer (1904), Lanzi (1909)
and Rais (1910), while the development of the mesoblast and the
lengthening of the tail bud was investigated by Swaen and Brachet
(1899, 1901, 1904). Sumner (1904) and Kopsch (1904) dealt in their
experiments with the morphological problem of the formation of the
embryonic body in teleosts, but Morgan (1895), Lewis (1912), Hoadley
(1928), Oppenheimer (19 3':) and Luther (1935) took a more physiological standpoint, and considered the determination of organ. systems,
the regulating pavers of the egg and organizing influences (Oppenheimer
and Luther).
This brief overview shows just how unclear is our knowledge of
gastrulation in teleosts and how desirable it is to acqu ire precise,
clearly proven data. I have undertaken new research using the vital
staining method on the advice of Professor Dalcq, to whom I express
sincere gratitude for the warm interest with which he has c ontinually
followed my research.
This work was done in 1933, 1934,and 1935. Preliminary results
were published after the first year and a more complete article
appeared in 1934. After some supplementary experiments performed in
1935, I am now in a position to publish a definitive study.
2 - Materials and Method
Eggs of the species Salmo iridaeus (rainbow trout) were used
Because of its size the trout egjis -neal for experimentation.
S iridaeus eggs develop more rapidly and are more resistant than
S fario eggs. However, some control experiments were performed using
eggs of the latter species.
This study is based on 180 experiments on s irida.eusand 30 on
Sfario. Since the latter brought confirmation of data without
providing any new information, they will not be mentioned again.
The vital stains were applied. with a piece of agar saturated
with Nile blue (neither neutral red nor Bismarck brown was suitable)
pressed into the uncovered blastodisk with a piece of glass. The
eggs were opened with two well sharpened watchmaker's forceps. After
the chorion was opened, the eggs were held in position in a small
depression in an agar medium that had previously been immersed in
216
a crystallizer. The use of Holtfreter's fluid during the operation
hastened the healing of any minor lesions. After a few hours, the
crystallizer was placed in a bath of circulating water. Owing to
this precaution, survival was sometimes quite prolonged. (Out of
curiosity, I kept some embryos until they hatched, the mark having
of course by this time long since disappeared.) Most often, however,
the eggs died fairly quickly because of lesions in the fine plasma
membrane surrounding the vitellus, even though the embryo itself
was perfectly normal,
Because the membrane is so very delicate and the consequences
of any lesion to it fatal, it was not possible to apply more than one
stain at a time. The stains had to cover a relatively large area because
they tended to disappear after a while and the slowness of development
necessitated observation for three or four days. For this reason. the
stains were not localized to one particular field but were spread out
over several. To obtain an accurate map, experiments dealing with the
same area had to be systematically repeated on several blastodisks
Often, it was the small variations inherent in the experimental process
that made it possible to establish the details precisely.
•
•
When the stain was applied it spread beyond the edges of the piece
of agar. After a time, however, it stopped spreading and the outline
could be traced with a camera lucida. Like all previous investigators
who have used this method, I was able to ensure that the stain did not
diffuse once it had stabilized and that it characterized only one group
of cells and its derivatives.
The eggs were examined several times a day by means of camera
lucida tracings. Trout embryos are transparent enough that stained
cells which have sunk into the embryonic mass can be Observed while
the latter 11 ...i_11 alive. Preservation Of the marks on hi.stolo"0l_
have been de.sirahle but tlum«rous efforts t.o
..c-rtainJ.y
scct:i.otls wo.:1__ u:'inl;
a varietyo of methods, f.a.ilecl complet.el.y.
achieve t1ii.n
We living embryo in this wa}r should be comb i.ned with
1`iappin2 ::,study of f;oon ;.-:ial microscope sections. In ttl:i.s expe.riment, either
The
•fi
Allen and li...in fluid or Te.llyeslii.c zky fluid was used for
ii_^^ i <o^l,
s
1
first two f1.i:?-t` give the. best results but when using t
fixation
r) ^:' ate the chorion during the F.irst quarter hour of ,.a.tion
or else it •4 _. > not be possible to separate embryo from the tholou;;l^.y
hardened yo.-^c.1.i:h` disadvantage of this is chat the embryo is easily
deformed, es •.:v:iall.y in the early stages of gastrulation, in which it
^y fluid, on the other hand, is good .f:or.
t'snicz.^
3_s spread 13_.^. .s11 •YAs the embryo becomes fixed i t separates comi7lC'.te-_y
j
1t13Cr03C01).-.C
t ^' ,;:i =^ •
from the y:..:-k, o ich h^zl'dens, and retracts. After it has bc.e.r_ fixed in
1y detach it from the plasma nl^^illhr.ane. s=.ira'oundini; the
sit-us on<. ^^::•:^d :: ;:
^>ci:.cr}°
yolk to fI_r:t: :.t ^:ntircl.y. This type of fixation i;_i.°ve5 sat.i5f
results on sLc:.:^n, although there is some tendency for the I.aYel"e> to
retract.
II
^Pt^,P.SOItiAL c3BSEVA`3'ï.ONS
1. - General outline of development:
on the basis of macroscopic
observation
7 f:e1t it would l)e helpful to present a complete set of drawings
v n^V
scr.l::si^^^
etc'cut.ed from in tot^o fixations before di
were
published
by
kler:lne;uy
(1888)
f^lit:113:^^11y
^rran c ^U
differ from the S Eaï; o eggs
e^;pc^r:i.inental I'est;li:s. Similar di a"
and Kopsch (1898). i^io^aevery S i r:i.d.r^au._ eggs
•
^h1-. be wel l to
-^^
1: T1 this respect it
no to
that
one _ 4:1.s11c^.:i
when l '-11
no point in
v(:
^ U rem ove
it in c, al_i.11E'.
,,
ai_ors ^l:lvi.sr .1.. 1^^-^_n
there is
the c.li;^Li.on from
the cg:
;4
,^7^if'^.
solve
the yo1_l.,
as e^ar.Iie;° i.Tlve^.1)_:.a
o 1.:.f.i.c s
t
,
the coa ;u.i.atc cl yo11 ^eft-^>
solution
forceps, it ise.asy to a:t_move.
colnpletel y.
218
•
studied by the latter authors in the timing of the envelopment of the
yolk, which occurs more rapidly in S fario than in S iridaeus, As a
result, at the time of complete clo-su-re of the blastopore (Figure 3M),
morphogenesis is more advanced in the latter species.
In all the following figures, the larger diagrams represent either
the isolated blastodisk, in the earlier stages > or the developing
embryo alone, in the inter stages. They are taken from camera lucida
drawings and are clone to the same scale. Beside each blastodisk or
embryo is sketched a side view of the whole egg so that the relationship
between the blastodisk and the yolk at each stage can be understood.
On the whole egg in Figure IA can be seen the protruding,
hemispherical disk of the blastula stage. Beneath the isolated and
enlarged disk can be seen the subgerminal cavity that has just formed
See photocopy of original
Figure 1 - External form of S iridanus embryos from in to:to
fixations . The larger diagrams7epresent isolated b1 -81-Siodiks
seen from above, the smaller ones, side views of the whole
pgg. A - blastula, B - young gastrula. C and D - young gastrulas
with terminal knot beginning to appear.
GASTRUL.-,,TION DES VERTÉ73ÉS
j. PASTEELS
8.
_Tarit), étudiés par ces auteurs, par la chron•logie de
stades représentés.
Sur l'oeuf entier de la ligure 1,A, on aperçoit le df›•.i,L
hérnisphéri ,..,..z, du stade blastula. Ce disque
:
A
,
.
(
•y
•
,.,
i.
/
\,.
-..
. ,
.
• .. •\ \ ,/
. • ,,
\
•-.
D . ....--.----77------• -......,._
,.
y
7
.,,
.......
.;...
\
.....
\ ,
;
••..•
1.
• •
.:
—
r;-(; 17:1
0. icunc
C. e's 0.
•
‘.1:;:r.brvon-: d.:
-
enticr. A.
avec
.1:
QI:
S
2 19
sous -germina le
et agrandi montr e car transparence la ca-,;ité
ventrale. (Nous ayons
cui vient de se creuser dans sa partie
minale est. bien
s)us-gezreorr'senté un crut" dent la cavité
l'orientation.
Comme
••dévelo7,pée, permettant aisém.ent
nous le verrons plus loin, 1 1 n'en est pas ainsi pour toutes les
représente un blas.l.odiso2..,:e qui s'est
• • •••••-'.:s.' pontes). La figure .1.,B
"étalé à la surface du vitellus. 17,:térieurennent, à part la
taille, ce stade ressemble fortement nu précédent. Cependant.
l'•,-r.,-..trén -lité
des coupes indiqueraient qu'au niveau de
a déjà c ri n,,,,e ; :c.,é. Uu stade
• • ...postérieure, la gastrulation
In formation des
Yastruintic.)n plus avancé se voit en I,C;
•
feuillets profonds n'est plus !imitée au bord postérieur
périph:.'rie du disque ; à noter
/;..- • mais s'est étendue à toute la
la saillie ni.édianc et postérieure : c'est le début d'apparition
• clu « nceud terminal ». Celui-ci se remarque encore mieux
sur la figure I,D. En avant de ce nœud, l'ébauche embryonelle se continue
..--"naire commence à se dessiner ; latéralement
ce st.cd..;
par le bord d'enveloppement
niveau duquel on trouve deux feuillets. Au milieu, la cayln.'
sous-germinale dont la voûte cellulair e s'arnincit de plus
dit Vin:ULIS.
- en plus, s'étend progressivement à la surface
où la plaque médullaire
". Un stade -analogue se voit en I ,E,
se dessine nettement en avant. Ln partie antérieure de
i termin::.1
cette plaque rrr'dul:alrP et 1-1 suiilie cia Pc-2,11c•
re2.1-: lateralement l'embryon
SII,`.:(1: de la
s accent
• ,• •
Cieux
sortes
de
cornes qui s'effilent dans un seprolng
Cri.
, " •
bord d'enveloppement Fort aminci du c:(5té ventral. La
figure 2(3 111011trC 1.111C éb:11.:Che embryonnaire avant à peu
. .
, p1.- s la rnérne fc.)rine que la précédente., mais beaucoup plus
•
allongée. Cette élongation s'accompagne d'un étirement.
Le vitellus est à présent à
-•: • pour l'embryon de la tiçzure.
fbrmes sont encore fort
par l'embr\- on.
•
.re
.. • imprécises.:Ytais un peu plus tard (ftg.2, 1) on voit appurai .
• les renflen:ents cérébr:::ux, les ébauches ocubires et.
• -•
..
Iement de la large ébauche médullaire, 0 paires de somitcs ;
i . • plus en dehors un liséré blancii:•Itre représ...snte le i nésubl.i.l.te
l'embryon
. • latéral. L'ébauche nuidullaire srsmbl , • s'étirer d
j. • ."
est plus Vite e:il
l'enveloppement du vitellus ;
chez 8. fario que chez S. iridacus ; il s'ensuit qu'au mon-.ent
de la fermeture complitc du blastopore (Fig. 3,M) !a 'inr:,."pl .,n(zérii:SC est plus avancée chez la dernière espèce.
Tous ces dessins reproduisent en grand le blastodisu..:e
isolé pour les premiers stades, l'ébauche • embryonnaire
seul e pour les stades ultérieurs ; ils ont été exécutés à
chambre cl-aire, au méme i...7 ,rossissernent, et sont donc
rerré•Triênie éeI,elle. A chaque figure est -annexé un schéma
sentant l'œuf entier vu de profil, ce qui permet de comprendre
les raDi-7 , rt"S ciu blastodisque et du vitellus a chacun de,
É
219
•
•
in its ventral part. (An egg with a well developed subgerminal cavity has
been drawn, for easy orientation. As we shall see fUrther on, the cavity
is not as distinct in ail eggs.) Figure 1B represents a blastodisk
flattened out on the surface of the yolk. 'rom the outside, apart from
the difference in size, this blastula looks much like the preceding one
show that gastrulation has already begun at the Howevr,sctinoald
posterior extremity, A more advanced stage of gastrulation can be seen in
1C. Here, the deeper Loyers are not limited to the posterior edge but
have spread along the periphery of the entire disk. Note the posterior
redian protruberance; this is the first indication of the terminal kne,
which can be seen to better advantage in 1D, In front of the . knot, - Vle
embryonic primordium is becoming apparent. Laterally it is continuous
with the enveloping border (very wide at this stage), where there are
two layers. In the centre, the subgerminal cavity, whose cellular ceiling
is becoming thinner, is spreading out on the surface of the yolk, An
analogous stage can be seen in 1E, where the neural plate is quite
distinct. The anterior part of the neural plate and the terminal kiflot are
even further developed at the stage shown in Figure 2F, Laterally the
embryo continues in two horns, one might say, which taper off into the
enveloping border, now very thin. ventrally. Figure 2G shows an embryonic
primordium much the same shape as that in 2F, but considerably longer,
The yolk is now half incorporated into the embryo, the form of which is
still imprecise. But a little later (Figure 21), the brain vesicles, the
primitive eyes, and lateral to the wide neural anlage, nine pairs of
somites appear. Further toward the edge, a whitish border represents
the lateç- al mesoblast, The neural anlage seems to become drawn out in
the embryo of Figure 3d, where there are fifteen or sixteen pairs of
•
See photocoy- of original
Figure 2
See Figure 1. E,F,G - Formation of the neural plate.
1.1 - Lengthening of the somites. I - Appearance of the somites.
somites. As 5e ne preceding stage, the anlageis swollen posteriorly, thus
forming the pr !. _-..Ative terminai knot, It continues laterally as two horns,
which constIte the dorsal portion of the enveloping border. In
Figure 3K, grt progress is evident in the differentiation of the
nervous systLn: and the primitive eyes. Otic vesicles have developed and
there are tw...ny-one pairs of somites. Behind the embryo, the blastopore
and smaller; it surrounds the inferior pole of the
is becoming
yolk somewhat off-centre. In Figure 3h only a very small, oval blastopore
is ev.ident J a 7îgdre 3M, the last stage we shall consider, the blastopore
•
1
•
22
4111■
!.:CY:3"...AST:QUES
CASTP.ULATION DES
• 7. PASTEELS
220
,
.
.••
/
:•:
,.
-j
r
••
'•-
' ••
tt.
.t,
I
'‘,. .
.
' 'I
i
•••.1
•'
i
i
;
..
. :
t.
:1•
N
• .i '
1
.
. i. l'-•
•
:4
,..
.. .
• :.
Lc blaCIrporc
la figure
à sa place, entourant le bourgeon caudal renfl-f;
est
de mésoblaste ventral. Un systny..: flPRUX
se voit: u:-tcrnunic-z
ds cupilics
pl:sscnlentscébr
cins,
ce
39 paires dc sontites
d'un cristallin, 33
stade.
cons:dérerons
staHc que nous
z
•
;...
.;
•
H
/
• I,
,
• ,.....—
5P
-:----'' ,
.\
If
\
S...
--,--.:-....-------;
'\:
---------...../.."-- ----1. •-• ''''
,r:
I. E. F, G. Fr..r.t -c.tti
F. 2. — Cf. Co..;..
•
1
' • ./
sor,lites.
16 paires (Ic somites,
15
l'on
voit
3,j,
ofa
,
de la figure
comme au s.racie iprécticlent
'
rebauclic
nlédullaire,
ternoter qtp::,
l'ormat:t airksi le nr_eud
toujours renflée en arrière
cornes
esi.,
latéralement en dt:ux.
minal, elle se prcionge
bor.c.I dC1\-Co
c)rtlon dorsale du
cnnstituent it
1:_tdifférencia :li or.
tal .g.rand i)rorés dans
montre.
oculaires :
La. fignrc. 3,1-ç.
nerveux et des ébauclies
systine
des fornaç.:s
21 paires dc
vési...:ules auditives et
des
de Plus
,..c
trou
on y
le blastopore se rti
l'ornt-)r.:,.opi,
En urrlire
ex.zentricnienient lc
en plus et entoure
petit, de rornie
derr.i ,:r
Itiquelle sucede bient_ût le
1
.
—
ettract,.:riseki Ezure. 3,
FIG.
3.
—
Cç.
1..3.
res...;:ve. du
K. L. Fr..•1-,Iut.1.:•..-: yrcv:...,,
M. La f..-,;Istrul2tion cst
En avant d'elle Sc trouve
avons vu que la téte es:
rep....re
,.1, excellent point de
le svnc..\,, ziunl vitellin centr:et qui persiste au pôle anim.al de
• signale' par ViRcH0\:.:-(1S95)
Egures qui montrent l'embryon
l'il;euf. I_ a. comparaison des
iridaciis le mode
sur le vitellus prouve que pour
du v tcikS tel ou•ii
. d'envelor.•tement :.,,:centriqu,'
par IOSCi (100 4 ) clans une
représené, pour Salino faric
4) est. valable aussi. Cette
ure reproduite ci-joint (Fig.
réunion 'a l'er.abryun
figure sell:arise en outre le mode Lle
ban.,1 d'env:-..ioppernent, tei qne
des parties latérales du
• ,
NI.J1..i:•.;
221
bas closed and in its place, outltning the tail bud swelling, is a ring
of ventral mesoblast. This stage is characterized by a closed nervous
e system with cerebral folds, optic cups with a lens, and thirty-eight to
thirty-nine pairs of somites.
See photocopy of original
Gradual closing of the
Figure 3 - Compare Figure J, j, K, L
blastopore. M - Terminn,tion of gastrulation.
As we have seen, the head remains stationary. In front of it is the
central vitelline syncytium, which persists at the animal pole of the egg
and is an excellent reference point, as Virchow remarked (1895). Comparison
of (-113.wings showing the embryo on the vitellus proves that eccentric
envelopment of the yolk, as represented for Salmo fario_by Kopsch (1904)
in a diagram reproduced herein (Figure 4) occurs in Salmo iridaeus too.
This figure also shows how Kopsch thought the edges of the enveloping
border met at the embryo. We shall see later what really happens.
•
•
222
Sc e photucopy of original.
Figure 4 — Mode of envelopment of the yol k by the Salmo fario
embryo. From Kopsch (1904).
2 — The blastula
Formation of the subgerMinal cavity
In the morula stage, the blastodisk consists of a solid ball of
cells, the blastomeres being separated by small spaces only. However,
shortly before gastrulation begins, a "subgerminal" cavity forms in the
ventral part of the disk it lies on top of the yolk and bas a thin covering
of cells.
Strangely enough, the subgerminal cavity is always identical in eggs
from the same roe but there are wide variations in its size and the manner
and time of its appearance in eggs from different roe. Figure 5 shows two
sagittal sections illustrating two extreme cases. In Figure 5a there is no
distinct subgerminal cavity, only a thin space separating the cellular mass
from the yolk. The blastodisk is already spread out and when gastrulean
invagination begins, the top of the vitelline mass will be largely covered
by the cellular mass. The ventral part of the disk is thinner than the
•
•
•
222
GASTRULATION DES VERTÉBRÉS .MÉROBL..ML-iROBL.-■ STIII
ST
j. PASTEr.d.S
KOPSC;-1 le concevait. Nous aurons l'occasfon de revoir plus
loin ce qu'il en est réellement.
-
29;
s'on extrémité dorsale. Sur une coupe sagittale de blastula
•- encore jeune, ion pourrait voir que les cellule.s a biastodisque
(oui, par son aspect extérieur, ne présente aucune différen-
7.• 1 i"..-/
"I
;:\
,,l'--;-...---__
2 .
`
, ,.:
t
I
-----n- -
FIG. 4. — Pdcicle d'enveloppement du vitellus par l'embryon de
tario. D'après kopscll (I904).
— Lu blastula
Creir.-4ement de lu eaviti3 sous-uenninale
Au stp.de morula, le blastodi,sque est constitué pur un
boutja cellulilire plein, les blastoméres n'étant séparés que
par de menus interstices ; mais nueloue temps avant le
début de la gatrulatiOn, il se Forme dans toute la partie
ventrale de ce disque une cavité « sous-germinale »
sur le vitellus et recouverte par une voûte cellulaire amincie.
Chose curieuSe, cette cavité sous-germinale, toti;outs
identique pour les oeufs d'une méme ponte, présente cependant de r(i)rtes va: . iatiOns de taille, *de mode et de temps
d'apparition pour les œulS de pontes dirférentes. La figurz,‘ 5
montre en a et b, deux coupes sagittales illustrant deux sas
extrêmes.
Sur la figure 5,e on ne voit pas de cavité sotts-germii
nerte, niais un mince interstice séparant la. masse cellulaire
du vitellus. Le biastodisque est déjé étale et au inciment eù
l'inVtgination gastruléenne débute, le sommet de la
est assez largement recouvert pur la masse isellul.:ire•
La partie veril- raié de ce disque est amincie par repp(.:z
• .7
.•
•
FIG.
•
— Deux
coures sagittales de blastulas : l'une
zerminale virtuelle (ci",, l'autre
cav:té. bien
cintion) sont beaucoup plus tassées du côté dorsal, tandis
;qu'ailleurs elles sont séparées par de nombreux espaces
_intercellulaires. Cet état persiste nu début de lu gastrularion,
comme on peut le voir sur la figure 3 ,a. Un peu plus tard
Ci,b), tout l'ensemble, du disque ; plus étendu., s'aplatit:à la surfas ,' du •%,•- iteilus. Les espace-; intercellulaires
les cellules S'1 , ;lornèrei“.- en une Concile dense,
naturellement plus mince en uv..-',ut et sur les côtés. Cette
évolution se présente, quelques variantes, sur environ
un tiers des pontes de Sa.TiTio irit/7.itisi. Elle est cons' ante
chez Saiiriofario, dinre de celle nue nous al! , ,, ns décrire
par une apparition plus précoce des mouvements d 'épibolie.
En 3,b, nous représentons une disposition que l'un refic , ): 1 tie
e plus souvenu chez S.
Le
iusqu'aUX premiers s::;.Ide: de ld
223
.
0
dorsal extremity. On a sagittal section of the young blastula one would be
able to see that the cells of the blastodisk (in which no differentiation
is apparent from the outside) are crowded closely together dorsally, while
See photocopy of original
Figure 5 - Two sagittal sections of blastulas (a) with a potential
subgerminal cavity (b) with a well developed subgerminal cavity ,
elsewhere they are separated by numerous intercellular spaces. This state
persists at the beginning of gastrulation, as can be seen from Figure 6a.
Somewhat later (Figure 6b), the entire disk, larger now, flattens out on
the surface of the yolk, The intercellular spaces disappear and the cells
agglomerate to form a dense layer, thinner in front and on the sides, This
mode of development, with a few variations, is displayed by about one third
of Salmo iridaeus eggs, it is the only one shown by Salmo fario. It differs
from the type about to be described in the earlier occurrence of epiboly,
Figure 5b shows the mode of development most frequently encountered in
Salmo ir:idaeus. The blastodisk remains almost hemispherical and continues
to protrude from the surface of the yolk until the early stages of
gastrulation. The anterior (ventral) portions of the disk have already
•
t,-u]-ea?1 inv<ïSiilai:âon Uegi.ns ïre.l^.
at this Point ` Gas
oveï• the Vite l lus .
Mc h th:i.nne
out
s P ï."cads
come^! the
^
-
^ me r.c^
of original
c^n1:L1181
the ^a
two emUx^'os from
otentâ_al subg,^rs with P
_ Sagittal sections of
of gastxu7-ation (eggs
Figure 6
is mosi:
the Uc'-h^.i^i.ng
.mina7_ cavity
at
oi_ng
T^T P e o{ sul?geï' dis^
cavit i.es) .
und^^r^;
eï
l`
a-s
,^^S
Lr.an^'
mile the U^_astG
ale a^ï,.^j
aUove
formation
Of
the
1aï'f^
there
rt each otller;
The mode of f some can ^ideï'ation,
of Ulastu1-a
a
aga ^-17..
dorsal 'r:i_b
halfi•
^- ?
cur:Lous and merits
ce11s are nev'ci t t^ht
^ çton:e.xes
em . ^s 1i1 he -`1 ïsjc and
1?1a
the
-c,
the bec- the
Uei:weenP] Ceus
c1 ea.vaSe itsIlc_ Gf
y._ one s:i.c
1
,s a1?,e.ar
, s P`'ï ce_
u
^^ mmetry th
t-, ny ir^'eguav
"U
are mol e xtulnr-:ous on eazl^7
of
^
^l
rt
?a
^
U:i1aL.^.L^
of s aces in Se zre.nt.'ral. 1
on sc.ct^•Giï
the sPa.ces
ni zaU^ e The first LpCll.cati -G17S7}1e
urded Uy
See 1?hotocGl?y
P
in eLc1v suixo
Ueconies ZecGs
lleï_.
to inove
corae de£inite.
i es comP
C^,11t. 1.l1Lle
avit.---^,
a^e closer toge '
` t. Chey s oon be
f orm t: ïaG c ]_e as the ce11-s
The floors of
ear7_y Gil^
disk mexge to
UY iitt
,^,t'
o1.lc,
U-^u:La'
".
Gf tlle y
gL.Ÿ'faCe^e,entea in
1,}ost: vent^al Poo lies on. the
ce1^.s. The cavitie^
el.ol?ed
1eP
iall.y in the
th cavl-^11.s I' the stage
tU.athe
'one 111G'L" e detr
Uack esPec
?erf.oY'ate so avitit.s
^oxUed.
is s To
two(
c aï ea) ^
(sh^_ded
the cavi t-i es 1^^ ,aït:s.tio.i
^ of the
ste'riorly
and tlïe central l"
the outline^ flooï° s Po
qguxe j al whx-ch shows theiï cellular
Ys
T
than the other•) ,
very
Of the }?ro^.Yt1C^.:Ll1^; r e111.aY^4 i^•i t^e
•
0
tXOn of
the
•
J. PASTEELS
- GASTRULATION DES vERTI.:-.I3RÉO ,\IÉT-0 -3LASTIQLIES
hémisphérique à la surface du vitellus — les parties antérieures (ventrales) du disque sont déjà fortement amincies.
L'invaginaticn gastruléenne débute bien avant que le disque
ne s'étale sur le vitellus.
225
munies d'un plancher cellulaire en arrière (partie hachurée),
reposant sur le vitellus en avant et communiquant entr'elles
/':-., nlement vers l'avant. Un trait à travers In figure 7a
indic-nie la coupe représentée en 75.
— Coupes sagittales de deux stades de d:but de gatruiution
uovenant d'une uLinie ponte ; oeufs à cavité sous-grminale
Le mode de formation de cette vaste cavité sous-germinale
est Fort curieux et mérite que l'on s'y arrête quelque peu.
Les ccliulés du blastodisque en segmentation ne sont jamais
serrées les une con LYt: les autres et il existe toujours entre elles
de nombreuses lacunes menues et anfractueuses. Ces lacunes,
ici aussi, sont plus nombreuses d'un côté du disque, et
comme dans le cas précédent, la moitié dorsale se reconnait
déjà très tôt sur coupes Ç.Iràcc au tassement particulier des
blastomères. Les traces . de symétric: bilatérale sont doac .
C-,
1. ,
très.pécoeEl isntbô.Ducéveral
du disque saillant, les interstices s'agrandissent pour former
deux cavités paires, entièrement entouréés de
Ces cavités vont s'a.Qrandir peu à peu, par un retrait continuci
des cellules, retrait surtout accentué du côté le plus ventral.
A cc niveau aussi, les planchers de ces lacunes vont d'abord
se nc•rforer (In cavité reposant ainsi sur la surface du viteltls)
et la cloison mito‘.:enne va se résorber. De cette façon.
nous ami ivon à un stade dent une reconstruction grui
a été faite sur la figure 7m. Nous -v voyons les contour:z des
deux cavités paires (l'une est plus développée oue l'autre)
FIG- 7- — a) Re.:,+1;(ruczirn
Cr'21-13 e 1- n=nt
la cavié
par de.pnfiltill,;
d=
c2nufaire
en blanc.
figaue a.
tJ
i7)
1
Cnntou:'!:
:
blatula au ,:our› (J1.1
cor:tu...11-s dz la ravit,: sour
niunic
ph,
co. upe pa , saut var le trait
la
Dans LI suite, ce:,. motp.'ements cellulaires vont se précipiter. Les planchers des deux cavités se rétractent de plus
en plus, de inénie que la paroi mitoyenne. Celle-ci forme.
225
Anteriorly they rest on the yolk and join onto each other. The line across.
Figure 7a indicates the level of the section in 7b.
See photocopy of original
Figure 7 — (a) A blastula during the development of the subgerminal
cavity. The dotted lines indicate the contours of the cavity; the
shaded area, the part that still has a cellular floor and the white
area, the part that touches the yolk directly. (b) Section from the
level indicated by the line through (a).
Subsequently, the cellular movements occur more rapidly. The floors
of the two cavities retract further and further, as does the median partition.
The latter juts into the subgerminal cavity for a time, then disappears. As
O
•
226
•
a result of all this activity, there forms the large subgerminal cavity in
the still protruberant blastodisk Of figures lA and 5a.
Thus, the blastula stage is characterized by much cell movement.
However, it occurs only in the deep parts of the disk. The author tried to
find signs of such movement at the surface using the vital staining method,
but in vain.
Fifty-three hours after fertilization, a heavy stain was applied to
the centre of egg 150 (Figure 8a), still in the blastodisk stage. Twenty
hours later the subgerminal cavity had formed and the mark had not moved.
At ninety-six hours, the stain was still in the centre of the blastodisk,
which was undergoing gastrulation. But by this time it was on the most
posterior part of the thin ceiling of the subgerminal cavity.
The surface of the blastodisk had thus remained immobile. The central
part of the disk and the neural areas immediately behind it undergo a
minimum of movement. The central area stained in this experiment is always
found in front of the head, and it eventually forms part of the head
ectoblast. Virchow (1895) noted this fixed point when he found that the
central part of the vitelline syncytium remains there throughout development
(as can be seen very well in the living egg).
Figure 8b shows that another area on the surface of the egg, which
undergoes intense morphogenetic movement in the course of subsequent
development, remains just as immobile during the formation of the subgerminal
cavity.
At fifty-five hours, a stain was applied off-centre to embryo- 153, but
not too close to the edge of the blastodisk. At seventy-three hours, the
subgerminal cavity had formed but the mark had not moved. At ninety-six
hours, when gastrulation had begun, the stain was stretching toward the
dorsal lip of the blastopore. It ended up in the nervous system, from the
brain to the hind part of the embryo; some stain was also evident in the
posterior notochord.
•
227
*
•
1
See photocopy of original
Figure 8 - (a) Stain applied to centre of blastodisk (see text)
(b) Stain applied further dorsally.
Vogt (1929) showed that in amphibians the marginal areas of the
egg move downward toward the vegetal pole and that this process continues
from the young blastula stage until the beginning of gastrulation. That
marginal material descends toward the future blastopore has been even
more clearly demonstrated in the Rana fusca egg, which Votquenne (1934)
•
•
Y
226
pendant quelque temps une sorte d'éperon saillant dans la
cavité sous-germinale, puis disparaît à son tour. Tous ces
mouvements cellulaires aboutissent à la formation de la
vaste cavité sous-germinale dans le blastodisque encore
saillant des figures IA et 5a.
Le stade blastula se caractérise donc déjà par de nombreux
déplacements cellulaires. • Toutefois, ils n'affectent que les
parties profondes du disque. Nous avons tenté vainement
d'en retrouver trace à la surface au moyen de marques
• colorées.
L'oeuf 150 (fig. 8,a). 53 11, après la fécondation, à l'état cl,:
blastedisque encore plein, a été muni d'une forte marque en son
centre. Vingt heures plus tard, la cavité sous-germinale s'est
creusée, la marque est restée rigoureusement immobile. On
la trouve encore à 96 h. au centre du blastodisque en pleine ;as" . trulation. Mais elle est cette fois-ci sur la partie la plus postérieure
de la voûte amincie recouvrant la cavité sous-germinale.
•
•
" . GASTRULATIOti DES VERTÉBRÉS MÉROhASTIQUES
J. PASTE.ELS
La surface du blastodisque est donc restée immobile.
Ajoutons d'ailleurs que cette partie centrale du disque ainsi
que les territoires neuraux situés immédiatement en arrière
sont soumis à un minimum de déplacements. La partie
Centrale marquée dans le cas présent se retrouvera cons, tamment en avant de la tête dont elle viendra en fin de
con!pte former une purtie de l'ectoblaste. VIRCHOW (1S95)
avait déjà repéré ce point fixe en y retrouvant, d'un bout :I
l'autre du développement, la partie centrale du syncytium
vitellin (ce qui se voit admirablement sur le vivant).
La figure S,b montre qu'un autre endroit de la surface d,.
l'oeuf, endroit qui sera soumis au cours du développement
ultérieur à des mouvements morphogénétiques intenses.
reste tout aussi immobile pendant Ic creusement de I::
cavité sous-germinale.
Une marque excentrique quoique n'affleurant pas le is:+rd du
blatoLiibtlite est placée sà la 55'" heure sur rembryon153.:\
la cavité sous-germinale est formée, ;a marque n'a pas
• A 9i3 h., la ., ., astrulation ayant débuté, la nième tuarqn.
vers la lèvre dors:* du bla.:Fmpu; - e (de se retrouve en fin d.:
dans le systéme nerveux, depuis le cerveau jusqu'en nr.riere
du corps em1)1'yorInaire, aussi un peu clan la chorde pcisti.:ur:..).
•
"i"
73h..
55h.
96 Ft.
'''•
FIG. S
. —
blastodisque ;
le texte.
Évolution d'u n e m a rq ue c ■ , lorée placée au centre du
Ie texte. h) Mar4ue si tuée plus dorsale ment, voir
VoGT (1929) avait montré chez les Amphibiens que les
territoires marginaux de l' oe uf subissaient depuis la blastula
jeune un mouvement progressif de descente vers le pi)ie
végétatif jusqu'au seuil de la gustrulution. Cette descente
des territoires vers le futur blastopore a encore mieux été.
.
.
• used to prove that the chordamesoblastic territory is equatorial at stage
8, with the notochord located mostly in the micromeres and the nervous
system close to the animal pole. In selacians there is an analogous descent
of dorsal material before gastrulean invagination begins (Van De Broek, 1935),
Ille
Thus,
movements"
blastopore
topography
there is nothing that corresponds exactly to the "pregastrulean
in teleosts. The descent of organ-forming territories toward the
occurs only in the deeper parts of the blastula. The surface
undergoes no appreciable change.
3 - Distribution of organ-forming territories on the surface of the blastodisk
at the beginning of gastrulation
By applying vital stains to various parts of the advanced blastula and
observing what became of the parts thus tagged, I attempted to determine the
prospective fate of the various territories of the blastodisk. The results
are summarized in Figure 9. I shall begin with a description and discussion
of this fate rap, followed by an account of the procedures and observations
conducted on several of the eggs, so as to justify the conclusions reached.
Organization of anlagen in the blastula
Figure 9 represents a blastodisk seen from above. Its convexity has
been ignored end it has been depicted as if projected on a plane. The
orientation la such that the lower part of the figure corresponds to the
dorsal median (or posterior) part of the disk.
•
•
In the middorsal zone we see a small area with vertical stripes. This
is the territory that will become the prochordal plate. It lies at the outer
edge of the netoehordal territory, which is fairly wide in the midline but
stretches out laterally into two thin horns. The notochordal territory can
be subdivided into the area thatawill form the anterior part of the notochord,
229
I.
•
•
.
which lies immediately in front of the prochordal zone and the area that will
become the posterior notochord, whose arrangement is more complex. The
or highest part of the wide midline zone corresponds to the central
11110 anterior
portion of the posterior not ochord while the two lateral horns will forms its
lateral parts.
See photocopy of original
LEGEND
1
2
3
4
5
6
7
extra-embryonic ectoblast
cephalic ectoblast
brain
chord
truncal ectoblast
truncal nervous system
somites
Figure 9 - Organ-forming regions on the surface of the blastodisk just
before gastrulation begins. The blastodisk is seen from above; its
middorsal or posterior aspect is in the lower part of the diagram.
Vertical stripes: pr(œhordal plate; dots: lateral and ventral
mesoblast; crosses: unprospected caudal regions.
•
•
An analogous, but inverse, arrangement is found in the neural territory,
a vast regular crescent directly in front of the notochordal anlage. The
228
j..PASTEELS
démontrée chez l'ceuf de Rana fusca depuis que VOTQUENNE
(1934) a prouvé que les territoires chordo-mésoblastiques
sont équatoriaux au_ stade V111, la chorde se trouvant en
grande partie dans les micrornères, le système nerveux
affleurant au pôle animal. Une descente analogue des matériaux dorsaux, avant le début de l'invagination gastruléeint, -2,
se retrouve chez les Sélaciens (VAN DE BROEK 1935).
Nous ne trouvons donc pas le correspondant exact . de ces
mouvements gr prégastruléens » chez le Téléostéen. La
deséente des territoires vers le blastopore n'affecte que les
- parties profondes du germe. En surface, la - topographie des
territoires ne subit pas de modifications appréciables.
GAST —WLATION DES VERTÉBRÉS RODLAS11QUE' '29
- suit : la partie antérieure de la chorde se trouve immédia, tement en avant de la zone préchordale. Quant à. la chorde
postérieure, la disposition en est plus complexe : la partie
antérieure, c'est-à-dire la plus haute, de .1a zone élargie
médiane correspond à sa partie centrale tandis que les deux
cornes latérales en _constituent les parties latérales.
•
:3.— IV:partition des territoires sur la suriaec du blastodisque
an' seuil de la Ljastrulation
En plaçant des marques colorées dans les parties les plus
variées _de la surface de la blastula avancée, et en suivant la
destinée des parties ainsi repérées, nOus nous sommes efforcé
d'établir la valeur prospective des différents territoires (lu
blastodisque. Les résultats en sont condensés dans le plan
de la figure 9. Nous commencerons par la description et la
discussion de ce plan, pour citer ensuite quelques protocoles
types qui en permettent la justification.
a) Le plan d'ébauches de la blastula
La figure 9 représente un blastodisque vu par sa face
supérieure ; i a été fait abstraction de sa convexité, il est
donc supposé projeté sur un plan. L'orientation est telle
que la partie inférieure de la figure correspond à la régiun
médiane et dorsale (ou postérieure) du disque.
Dans cette .zone média-dorsale, nous voyons une mias.c
région hachurée verticalement : c'est le territoire de I.t
plaque préehordale. Ce territoire se trouve à l'extrémité
marginale d'une zone assez épaisse sur la ligne médim...,
mais qui s'effile latéralement en deux cornes amincies :
il s'agit du territoire ehardal. Celui-ci Sc répartit eamine
FIG. 9. — Plan des ébauches à la surface du bbstodisque au seuil
de la gastrulation. Le blaçtodisqu.: est vu d'en haut ; son point
dorsal ou postérieur est plucé vers k bas. Hachures vertizaleS :
préc.hordale ; ch. :
: sou. soiniles pointillé. laine.; latérales
et mésol:llust ,é ventral ; croix : régions caudales non prospe, -té.: ,
nerv.t
. : syistéine nervei.:x tronz.11:cery. : ,:;7VC:1:1 ; J.
céphalique ect. tr. : ccl ,lblaste trorical ect. extra-1'r. :
ectiklaste éxtra-embryonnair.:.
Une disposition analugue (mais inversée) se retrouve dans
le territoire neural, vaste croissant régulier, cerné d'un
contour plein en avant de la chorde. La partie adtérieur‘.:
230
ge
nterior central part, outlined by the dotted line, corresponds to the
rai, the posterior central part to the floor of the neural tube, and
the wings of the crescent to the sides and ceiling of the tube. The
neural crescent terminates on either side in the undefined material of
the tail bud (crosses) and it stretches much further ventrally than the
notochord-forming material.
The entire anterior central part of the disk is occupied by a vast
ectoblastic area, lying above the subgerminal cavity. The dotted outline
in the dorsal central part of this territory indicates the cephalic
portion of it, which lies immediately in front of the brain anlage. In
the horns of the neural crescent we find, also set off by dotted lines,
two areas of truncal ectoblast. The ectoblastic field in front of the
cephalic and truncal zones forms the extra-embryonic area of the vitelline
sac.
Returning to the edge of the disk, outside the notochordal anlage and
the horns of the neural crescent we find the somite-forming area. The
outlines of the first twenty-three pairs of somites have been traced; the
rest merge into the unprospected zone of the tail bud. Note the arrangement
of the somites. They are oblique and they become markedly more so in the
lateral and lateroventral areas, In the outermost part of the disk,
beginning beside the eighth somite, there is a narrow, finely dotted
crescent with its centre ventrally, from which the lateroventral
will form.
The mesoblastic_material of the intermediate mass, which forms the
blood, the anlage of the pronèphros -and the anlage of the Wolffian duct
are not represented. It was not possible to distinguish them in the
living blastula. However, we know from the work of Swaen and Brachet (1899,
1901) that the hematopoietic material originates in the external parts of
the somites. Will the nephrogenic material be found between the somitic
area and the lateral mesoblast? Or will it be found in the internal marcrinal
c
zone, where Vogt found it in Aiiurea? (1929)? Not until we are able to preserve
the stained tissue on section will we be able to answer these questions.
More important and easier to answer is the question of the endoblast,
an extremely thin layer of cells that I managed to glimpse in only a few
of the living blastulae. As explained further on, careful examination of
sections has led me to adopt a point of view similar to that of Sumner
(1904). The endoblast is formed by invagination of the superficial layers
covering the dorsal and dorsolateral parts of the marginal zone. Since it was
not possible to trace the outlines of this anlage to a reasonable
approximation, it does not appear on in Figure 9.
How accurately does this fate map represent the distribution of anlagen
on the surface of the disk? To what extent are the data given here precise,
to what extent approximate? As stated earlier, the vital stains were observed
from the blastula stage until they had become incorporated into the organs
of the very young fish. The above fate map is therefore a true representation
of observed events, not a mere sketch of what is thought to happen. However,
because of the difficulty of applying stains to the teleostean egg, I cannot
claim to have established as strictly accurate a map as Vogt was able to
produce for amphibians. Figure 9 accurately depicts the anterior boundary
of the nervous system,*the relationships between its cephalic and truncal
parts, the lateral borders of the median notochordal territory, and the areas
where the notochordal horns come to an end and the lateral mesoblast begins.
Similarly, the arrangement of the first twenty-three myotomes is an accurate
representation of what was observed. On the other hand, since the size of the
various territories could not be clearly established they are drawn to an
approximate scale only. Although it is certain that the notochordal horns
become thinner on the sides, their size may not be exactly as shown; the
same applies to the material of the lateral and ventral mesoblast.
232
•
•
The uncertainty is even greater for the ventral areas (caudal material
and mesoblast) because there is no means of marking them directly. This
anterior part of the enveloping border is stretched so much and unites
with the embryo so late that any stain applied to it gradually fades out.
However, direct staining of the enveloping border at an advanced stage
is impossible because it is so narrow and because the vitelline surface
is so delicate. Mapping of the caudal areas will have to be done on a
species more suitable for this purpose than the trout. The best I could
do was indicate with crosses the area where myotomes, nervous system and
embryonic ectoblast probably end. Indirect evidence to be considered below
shows that these axial territories do not continue into the ventral
marginal zone.
Thus, we are not in a position to decide whether the caudal material
develops from a bud of indifferent cells, as Vogt thought (1926,1929), or
whether the various organ-forming territories of the tail are merely
prolongations of those of the trunk and are brought into position by the
same morphogenetic movements, as Bytel (1931) showed to be the case in
amphibians.
Further experiments on more suitable teleostean eggs are, I repeat,
desirable.
(b) Details of the experiments
e- •
•
(I) Boundaries of the notochordal territory
1
Seventy-one hours after fertilization, a
Embryo 126 (Figure 10)
stain was applied to the edge of the disk, spilling over onto the_
vitelline syncytium. Despite the absence of a clear-cut subgerminal cavity,
On all the diagrams of the experiments stains in the superficial layers are
indicated by dots, stains in the deep layers by horizontal bars.
I
I
'
imehich would have made possible precise localization, the stain was applied
II,xactly at the midpoint of the posterior edge through a fortunate chance.
At eighty-nine hours, in the young gastrula, the stain had elongated and
partially invaginated. At 114 hours there was a wide diffuse stain in the
front part of the deep layer, which narrowed down to a thin band toward the
back. This stain stretched to the hind part of the embryo, where it
See photocopy of original
Figure 10 - Stain in the posterior marginal zone. Dots: surface
stain; bars: invaginated stain.
reflected back into the posterior part of the superficial layer. At 138 hours,
when the organs were forming in the anterior part of the embryo (stage .7),
the prcchordal plate was stained and so was the entire length of the
notochord and the posterior part of the nervous system.
It can therefore be concluded that (a) the dorsal marginal zone is
occupied by the notochord-forming territory in front of which, up against
•
•
232
à
J.
PASTEELS
matériel des lames latérales et du mésoblaste ventral.
Pour les territoires ventraux (matériaux caudaux et mésoblaste) l'incertitude est d'autant plus grande qu'il n'y a
pas moyen de les fepérer de façon directe. Cette partie
antérieure du bord, d'enveloppement est soumise à un tel
étirement et se réunit tellement tard à l'embryon que toute
marque faite à ce niveau se dilue insensiblement et s'efface.
D'autre part, la coloration directe du bord d'enveloppement
à un stade avancé est impossible, à cause de -la minceur
...ex-trème de ce -bord et surtout de la fragilité de la surface
vitelline. La prospection des territoires caudaux devrait
être faite sur une espèce plus favorable que la Truite. Nous
avons dit nous borner à indiquer par des croix, la ione oit
m ■,, otomes,. système nerveux, ectoblaste embryonnaire
doivent vraisemblablement Sc terminer. En effet, nous
pouvons affirmer, en vertu de preuves indirectes que nous
verrons plus loin, l'inexistence de ces territoires axiaux dans
la zone marginale ventrale.
Nous- ne sommes donc pas en mesure de pouvoir décider
si ces matériaux caudaux proviennent d'un bourgeon de
cellules indifférentes (conception de VOGT (1926, 1929))
ou si, comme Mue BYTEL (1931) l'a démontré chez les
Amphibiens, les divers territoires présomptifs de la queue
ne sont que le prolongement de ceux du tronc et se mettent
en place par les mêmes mouvements morphogénétiques. •
De nouvelles expériences sur un œuf de Téléostéen plus
favorable restent, .répétons-le, désirables.
b) Détail des expériences
;
•
•
GASII2ULATION DES vERTÉBAS'MÉROBLAST1QUES
233- •
nette, permettant un repérage précis, un hasard heureux a
localisé la coloration exactement au point médian et postérieur.
89 h. : gastrula jeune, la marque s'est allongée et invaginée en
partie. 114 h. : dans le feuillet profond, on trouve une coloration
large et diffuse en avant, limitée à une bande longitudinale en
arrière ; cette coloration s'étendait jusqu'en arrière de l'embryon
• (
d
:1
I CI
/
1
r-
(
)
\
,, ss
-=.
)
u),,.\--„, J•e
_1:., -____, /
. :
t
/
Q
1
• •
1
)
•••••
"".••"
/
FIG. IO. — Marque color:;e dau: Ia zonc-: marginalc postérieure.
: marque en surface ; tirets : maxque invaginée. Cf. texte.
Limites du territoire chordal.
Ili) (1 ). — Une marque débordant sur le
Embryon 126
syncytium vitellin est appliquée à la marge du disque, 71 h.
après la féc t m..lation. Malgré l'absence de cavité sous-germinale
,
oit Me se réfléchit dans la partie postérieure du feuilkt Superficiel.
138 h. les organes se constituent en avant de l'embryon (stade ;
colurat.ion de la plaqu:2 préchordale, Cle la chorde ilepuis l'avant
jusqu'à son extrémité postérieure et de la partie postériePre
du système rk:rvenx.
ures reproduisant des protozoles d'expeirienees,
(') Sur tut es ces
les color.ttii:n cl.:s feuillets st:perficiels.sontleprésntées en
cèn;_s d e s it uinet..; profonds par des hachures transversales.
Nous pouvons donc conclure : a) que la zone marginale
dorsale est occupée par des territoires chordaux, précédés,
6,.)
the yolk, is the prcthordal plate (b) that even before gastrulation
begins there are truncal territories on the midline, stretching to
the posterior extremity of the notochord and the neural tube, and
(c) that the marginal zone is quite narrow because even a small stain
spills over onto the prospective nervous tissue.
We can assume, then, that a mark in the same area, a little
further from the edge of the disk, would eventually be found in the
notochord, in its posterior part only, and would stain a greater area
of the nervous system than the mark applied to embryo 126. This is
illustrated below.
See photocopy of original
Figure 11 - Stain above the posterior marginal zone. Dots: surface stain;
bars: deep stain. See text.
Embryo 141 (Figure 11) - At sixty-six hours, a well developed
subgerminal cavity made possible precise application of the stain. The
roundish mark was some distance from the edge of the disk dorsally,
considerably to the right of centre. At ninety hours, while the
anterior part of the mark remained immobile, the posterior part was "
actively lengthening and beginning to invaginate. By ninety-eight hours,
the movements of extension and invagination had become more intense.
Note the blue banding in the posterior median portion of the deep layer.
•
234
PASTEELS
•
eux-mêmes, tout contre le vitellus, par de la plaque préchordale ; '1,) qu'il existe sur la ligne médiane, dès avant la
gastrulation, des territoires trorzcaux s'étendant jusqu'à
l'extrémité posté.ricUre de .1a chorde et du tube neural ;
c) que la zon t . marginale est assez étroite, puisqu'une marque
assez limitée déborde sur le territoire nerveux.
On peut donc présumer qu'une coloration de la métne
région,s'écartant un peu plus du bord du disque, ne colorera
que la partie postérieure de la chorde mais tille plus grande
étendue de système nerveux. En effet :
;
.
,
0
GASTRULATION DES VERTÉBRÉS,MÉROELASTIQ ..«.
235-
•
feUillet profond. 113 h. : coloration de la chorde, à l'exception
e la plaque préchordale. Dans le système nerveux, nous trouvons
une large ligne colorée occupant tout le centre droit de la plaque
médullaire et qui s'épanouit dans urr2 coloration d'ensemble
du cerveau antérieur droit.
Conclusions : a) en arrière des territoires cérébraux, il
existe des territoires destinés au tronc (plancher du tube
médullaire et chorde) ; b) nous pouvons préciser les limites
antérieures de la plaque médullaire et du système nerveux
céphalique.
-1 -1 6 h.
\
93h.
FG. II. --.Marquc c:olorée cn haut de la zone marginale
: C...loration en surface ; hachures : coloration e.n
Cf. texte.
Embryon HI (r.ig. 11). — 6'311. : une belle cavité sous-germin: ..ic
permet un repéragc précis. La marque arrondie, à quelq;:::
tance cl:. la tnar.r,'Llu disque, occupe une zone dorsale i.l.:hortIo...nt
fortemcnt ..;t.n' la droite. 90 it. : tanJi que .la partie antériLure
dn reni2re re-;te immobile, la partie postérieure est sounli:,e à
:
t counmnc. à s'invaginer. 98
extension intn
vin:ntd'extension ct d'invagination deviennent plu.-z
t
Nutoiu lt: trait bleu dan-; la partie médian....
92h.
FtC. 12. -- AIarque cnlorée dans la zone marginale latérale. Piri-
tli! : coloration en surface : hachure, : coInratio -n en prof,)ndeur. Cr_
texte.
Embryon 103 (fig. lz). — 73 h. : marque alion... , éc le
11 ■ )ril du disque. lin. e flèche indique la poition de la lèvre
du blastopore, note dès son apparition (la cavité sou::-germin:Ile
n'éhit pa -; visible). 92 h. : la marque s'allonze vers la
médiane et s'im.T.ginc. 1113 h. : tant dans le feuillet profon,l qth:
dans le feuilkt
leS parties colorées se rang‘:iit le
long d i; corp.:, embryonnaire. Après la mise en pl:hse dt:-; orgar.Ls
for pro.chorclal plate.
113 hours the chord was stained cept
exr the
the
In the nervous system there was a bistcltoathe right1ofrCentre and
entire length of the neural plate j
spreading out to colour the right anterior brain•
behind the brain anlage, there are terri.tories
Conclusions: (a)
of the neural tube and
from which the trunk will develop (floor
notochord) (b) the anterior limits of the neural plate and the
cephalic nervous system may be defined.
See photocopy of original
Figure 12 - Stain in the lateral marginal zone. Dots: surface stain;
bars: deep stain. See text.
-three hours, stain was
- At seventy
Embryo 103 (Figure 12)
of the di.sk• An arrow
ore s noted as
applied for some distance along the edge
indicates the position of the dorsal lip of the blastopore
soon as it appeared. (The subbgerminal cavity was not visible.)
By
and h
m
tainidline
the
along the side of
ninety-two hours the stain hhôt^^^'^herewwas
begun to invaginate. At 116
and the superficial layers. Once
arent (no drawing was made)
the embryonic body in both the deep
the beginnings of the organ systems were app
0
236
the stain could be seen in the anterior and middle somites, in the
lateral part of the truncal nervous system and in the posterior
notochord.
Conclusions: The prochordal plate and the anterior part of the
notochord were not stained, but there was stain in the anterior
myotomes and the posterior notochord. A stain applied at one spot
on the edge of the disk therefore affected organs at different
levels in the embryo. One would think that of these organs, the
most anterior would have been the first to invaginate and therefore
that they would have developed from the material closest to the edge
of the disk. This indicates that the lateral boundaries of the
prPehordal plate and the central part of the notochord are found
inside the area covered by stain 103 but that the notochord-forming
territory extends laterally between the marginal somite-forming
territory and the neural area.
How far does this notochordal horn extend?
Embryo 144 (Figure 13) . - At seventy-two hours stain was applied
for some distance along the edge of the blastodisk, stretching from
300 to 100 0 from the posteromedial extremity (see diagram). The
vitelline syncytium was stained, so that it could be used as a
point of reference in succeeding stages (see crosses on drawing in
Figure 13). At ninety hours, movements of convergence and invagination
were noted (see below). By 114 hours the mark extended along the
side of the embryonic body, partly in the superficial layer and
partly in the deep layer. At 130 hours there was stain in the lateral
mesoblastt and anterior and middle areas of the body, in thë outside
edges of'domites 5 and 6, throughout somites 7 to 14, in the inside
edge of somites 15 to 20 and in the posterolateral notochord from
the area of somite 19 to the posterior part of the terminal nodee There
was staining of the nervous system laterally from somite 6 to the
hind part of the embryo.
Embryo 98 (Figure 14) - At seventy-eight hours stain was applied
to the edge of the blastodisk, extending from 35° to 95 ° from the
middorsal point. At 118*Iours invagination was noted and there was
stain on both planes of the right lateral enveloping border. By 118
hours the stain extended along the embryonic body, partly in the
superfiçial layer and partly in the deep layer. Note the extremely
oblique inside edge of the invaginated part; note also the area of
stain persisting on a central ring formed by the vitelline syncytium.
At 156 hours there was stain in the anterior lateral mesoblast, the
middle myotomes(observations were not as precise as in the above case),
•
*
_
Should this be ninety-six hours? (Tr)
See photocopy of original
Figure 13 - Stain in the lateral marginal zone. Dots: stain on the
surface or in the -nervous system; bars: stain in chorokamesoblast;
crosses: stain in the vitelline sysncytium. See text.
in the lateral parts of the nervous system and in the , notochord posteriorly
0
However, a stain applied between 90 0 and 145 from the middorsal
point did not affect the developing notochord.
•
236
.
J. RASTEELS
(le dessin n'a pas été fait), la marque se retrouve dans les somites
antérieurs et moyens, dans la partie latérale du système' nerveux
troncal et dans la chorde postérieure.
'Conclusions La plaque préchordale ainsi que la partie
antérieure de la chorde sont exclues de la coloration. Celle-ci
affecte les myotornes antérieurs et la chorde postérieure.
La . même marque a donc touché, au même niveau de la
marge du disque, des organes qui sont situés à des hauteurs
différentes dans l'embryon. Or, il est à penser que de ces
organes, les plus antérieurs seront les premiers invaginés,
doa situés le plus vers la marge du disque. Cela nous permet
de conclure que les limites latérales de la plaque préchordale
et de la partie centrale de la chorde se trouvent en z dedans
de la marque 103, mais que le territoire chordal se prolonge
ratéralement entre les territoires s.omitiques marginaux et
la zone neurale.
Jusqu'où se prolonge cette .corne chordale ? .
Embryon 144 (fig. 13). —72 h. : marque marginale très allongée
s'étendant de 30" à 100n de l'extrémité postéro-médiane. Le syncytium vitellin est coloré, ce qui sert de repère aux stades suivants
(en croix sur le dessin 90 h. de la fig. 13). 90 h. : mouvements de
convergence et d'invirgination (cf. plus loin). 114 h: :la marque
se range le long du corps embryonnaire, en partie dans le p!,:m
superficiel, en partie clairs le plan profond. 130 h. : coloration
des lames latérales de la partie antérieure et moyenne du corps.
de l'extrémité exicrne des soinites V et VI, dessomites V11 à XIV
en entier, de l'extrémité interne des sumites XV et XX, de la
Partie postérieure et latérale de la chorde depuis la région clo
snmite XIX jusqu'à 111 partie postérieure du nœud terminal.
Coloration du système nerveux latéral clepuib le somite
,iusqu'en arrière.
•
14). — 78 h.: marque marginale de 35 à
Embryon Ç..18
du point mét.liodurl, 118 h. : invagination : colaration sur ,.;eux
plans du bord d'envoppemcnt latéral droit. 118 h. :
rangée le long du coi:ps embryonnaire, en partie siturfici.,Ile.
extrême du eonlour .
en partie profonde. Remarquon's
partie
invaginée:
notons
aussi
le repère
de la
inter
persistant sur un anneau central formé par le syncytium vitJ
156 h. : coloration des lames latérales antérieures, des nlyzo. ,.s
GASTRULATION DES VER:TÉSRÉS MÉROBLASTIQUES
; t7'
•
Q! /
I
,
130h
' Fie. 13. —
Marqu2 dans la zone marginale latérale. Pointillés :
marque en surfa,:e ou clans le syste.me nervrux ; hachures : cubration
du chordo-m,:sobiast e ; cmix : coloration du Syncytium vitellin.
Cf.
texte.
moyens (l'observation n'a pas été aussi précise que dans le cas
précédent), des parties latérales du système nerveux et de la
chorde en arrière,
Au contraire, une niacque s'étendant de 90u 5 145' , ne
touche plus l'èbaucile chordale :
See photocopy of original
Figure 14 - Stain in the lateral marginal zone. Dots: stain on the
surface or in the nervous system; bars: stain in the chordamesoblast;
crosses: stain in the vitelline syncytium. See text.
Embryo 99 (Figure 15) - At seventy-seven hours, stain was applied
to the edge of the blastodisk, so that it extended from 90° to 1500
from the middorsal point (indicated by an arrow). At ninety-six hours
there was stain in both layers of the left enveloping border laterally
and posteriorly. By 120 hours, movements of convergence had brought
the stain up against the embryonic body and the posterior left quarter
of the enveloping border was stained. At 144 hours, the stain was
beginning to become incorporated into the posterior part of the
•
.
J. PASTEELS
238
GASTRULATION DES VERTÉBRÉS MÉROBLASTIQUES '9
.
FiG. 14. — Marque dans la zone marginale latérale. Pointii:
marque en surfa c e. ou dans le syst:me nerveux; hachures : coloration
du chordo-mésoblaste.: zrolx : coloration du syncytium vitellin. Cf.
textz...
77 h. : marque marginale s'ételidaiit
Embryon 99 (fig. 15).
de 90 ;à 150" du point inédio dorsal (indique par une
h. : coloration des .letix feuillets du bord d'enveloppement 96
latéro-po:;térietir fauelle. 12 1 ; h. : les mouvement 5 dc
out entrain a marque jusque contre lu corp,-;
le quart postt...rieur gauche du bord d'enveloppement est
144 h. : la marque commence S s'intégrer dans la partie
■
v
-
-
12.0!1.
FIG. 15. — Marque en haut de la zone marginale
Pointillé : coloration en uface; hanures : coloration du chor.'.0mésoblaste. Cf. texte.
.
239
See photocopy of original
Figure 15 - Stain on the top part of the anterolateral marginal zone.
Dots: stain on the surface; bars: stain in the chordlmesoblast. See
text.
•
embryonic body. By 166 hours, there was stain in the middle and
posterior _lateral mesoblast, the outside parts of somites 14 to 20,
throughout the following somites and in the still unsegmented
posterior mesoblast. A small triangle of stain also appeared in
the most posterior and lateral part of the neural plate. There
was no stain in the notochord.
In view of this, the lateral extensions of the notochordforming territories must come to an end less than 90 0 from the
middorsal point.
(ii) Nervous system
•
We already know the anterior limit of the brain anlage from stain 141 (Figure 11). The posterior neural boundary, which merges with the
anterior border of the notochordal territory, is less well defined
but it may be traced by looking at two stains, such as 126 (Figure 10)
and 141 (Figure 11). We also know from stain 141 that the brain anlage
is continued posteriorly in the midline by neural material corresponding
to the central part of the neural plate, and thus to the floor of the
neural tube. The stains applied laterally (144,,198; 199 figureS 13,
14, and 15) show that the neural territory extends on both sides into
a narrow area which lies above the notochordal horns and forms the
lateral and posterior parts of the spinal cord. Stain 99 (Figure 15)
indicates that the neural horns stretch further forward than the
notochordal horns.
it remains to define the lateral boundaries of the brain anlage and
the endpoints of the two neural horns that extend ventrally.
Stains 156 and 157 resolve the first question. They were applied
to closely corresponding areas of two blastodisks, laterally and at
some distance from the edge,Stain 156 was closer to the edge than
stain 157.
Embryo 157 (Figure 16) - At seventy-nine hours a stain was applied
laterally, at some distance from the edge of a disk with a well developed
subgerminal cavity. By ninety-six hours the dorsal part of the stain
had undergone convergence and extension; the ventral part had not moved.
O
By 120 hours there was stain in the cerebral and lateral neural areas
and the adjacent ectodermal areas. In the deep layer, the mesoblast
was stained. At 145 hours, despite a lesion of the enveloping border
which fortunately affected only the posterior part of the embryo, there
See photocopy of original.
Figure 16 - Stain above the lateral marginal zone. Dots: stain on the
surface; bars: stain in the deep layers. See text.
was clearly stain in the posterolateral midbrain, throughout the right
half of the medul1a9 and a narrow coloured strip in the right lateral
part of the neural tube. The corresponding ectoblast was also stained.
Embryo 156 (Figure 17) - At seventy-nine hours there was a well
developed subgerminal cavity. A stain was applied 90° from the
middorsal point at some distance from the edge. At ninety-six hours the
mark had extended slightly toward the enveloping border and the dorsal
•
240
j.
PASTEELS
rieure du Corps embryonnaire. 166 h. : coloration des lames latérales moyennes et postérieures, de la partie externe des sornites
XIV à XX, des somites suivants dans leur totalité, ainsi que
du mésoblaste postérieur encore insegmenté. On y trouve
également une petite tache colorée triangulaire dans la partie
toute postérieure et latérale de la plaque médullaire. Aucune
coloration de la chorde.
Les prolongements latéraux des territoires chordaux.doivent donc s'arrêter avant 900 du point média-dorsal.
pi) Système nerveux.
c-onnaissons déjà par la marque 141 (fig. 11) la
• limite antérieure du territoire cérébral ; la limite neurale
postérieure (se confondant avec le bord antérieur du terri• _.
toire chordal) est moins bien définie niais peut être tracée
grâce à la combinaison de marques telles que 126 (fig. 10) et
- 141, (fig. 11). Nous savons aussi par les résultats de la même
marque que sur la ligne médiane le territoire cérébral se
continue en arrière par du matériel médullaire correspondant
à la partie centrale de la plaque neurale, donc au plancher
du tube nerveux. Nous avons vu également au moyen des
marques latérales 144, 19S, 199 (fig. 13, 14, 15) que des deux
côtés, le territoire neui"-al se prolonge par une zone vraisemblablement assez mince, situé au-dessus des cornes chordates
• et formatrice des parties latérales et postérieures de la
moelle. La marque 99 (fig. 15) indique d'ailleurs que ces
cornes neurales dépassent vers l'avant les cornes chordales.
Il nous reste à préciser 'les limités latérales du territoire
cérébral et les limites extrêmes des deux cornes neurales
qui s'étendent vers le côté ventral.
•
GASTRULATION DES VERTÉBRÉS MÉROBLASTIQUES
coloration des parties cérébrales et médullaires latérales et des
parties ectodermiques adjacentes : en profondeur le mésoblaste est
également tOuché. 145 h. : malgré une lésion du bord d'envelop-:
qui n'intéresse heureusement que la partie postérieure pernt
•
•
(
ç
79h.
–
FIG. 16. — Mnrque en haut de la zone marginale latérale. Pointillé :
coloration en surfa - e ; hachures : colo:ation prrdondc. Cf. tete.
Les marques 156 et 157 viennent résoudre la premiére question.
Elles ont été établies en des régions très voisines : latéralement
et à quelque distance de,la marge du disque. La marque 15 5 est
située plus en dehors que 157. '
de l'embryon, nous voyons nettement une coloration de la partie
latérale et postérieure du cerveau moyeu, de toute la moitié
droite du bulbe et d'une mince zone latérale droite du tube
113). — 70 h. : marque latérale à quelque
Embryon 157
distance de la marge d'un disque à belle cavité sous-germinalc..
Ii h. : mouvement de convergence ut d'extension de la parti.,
dorsale de la marque, 1a partie ventrale restant en place. 120 h. :
Embryon 156 (fig. 17).
79
: cavité. sous-germinale tais
nette. Marque à 90" du point médian dorsal à quelque distance
de la marge. 03 h. : la marque est légrement étirée vers le bord
médullaire. L'ectoblaste correspondant est également coloré.
—
region. At 120 hours and 145 hours the stain was gradually becoming
incorporated into the embryonic body. By 168 hours there was stain in
the lateral part of the nervous system, in the middle and posterior
'regions of the trunk behind the thirteenth somite, and in the adjacent
éctoblast and mesoblast.
See photocopy of orginal
Figure 17 — Stain above the lateral marginal zone. Dots: stain on the
surface; bars: stain in the deep layer. See text.
The external boundaries of the brain anlagen thus lie behind stain
156. They were affected by stain 157, which made it possible to
indicate them on Figure 9.
•
243
J. PASTEELS
GASTRULATION DES VERTÉBRÉS e420BLASTIQUES
d'envelopPernent et vers la région dorsale. 120 h. et. 145 h. : la
marque atteint progressivement le corps embryonnaire. 163 ii. :
• S'il est facile de colorer, à l'aide de marques situées un
peu en arrière du centre du disque, exclusivement une partie
de l'ébauche cérébrale, il est cependant presque impossible
de toucher le tube médullaire sans atteindre en même temps .
: ce quilecrvau,pnmqergialtédn
• indique que la zone médullaire est h ce niveau assez mince.
Il n'en est pas moins« vrai, répétons-le une fois de plus, qu'elle
contient du matériel capable de s'étirer jusque tout en
rrière de l'embryorz. Ce matériel médullaire étant exclusivemen t. destine au plancher de la moelle, se prolonge des
deux côtés par les deux cornes neurales destinées' aux parties
latérales de la plaque médullaire. Nous retrouvons ces cornes
:dans les marques 98 (en même temps que la corne-chordale)
et 99 (fig. 14 et 15). Ce dernier cas montre que la partie
effilée du territoire nerveux dépasse vers le côté ventral
l'extrémité de la corne chorciale.
• Nous avons déjà dit qu'il n'est pas possible de précise,
avec exactitude la limite ventrale de ces cornes. Les me
apposées à cette partie du disque subissent, au cours Lic
l'enveloppement du vitellus, un étirement intense, ce qui
les rend à peine perceptibles ; de plus, comme le moment
où ces parties ventrales du bord d'enveloppement s'invaginent est extrêmement tardif, le piqueté bleu pàlit et
s'efface progressivement. Nous verrons plus loin que les
parties antérieures du disque peuvent être considérées
comme purement mésoblastiques (mésoblaste ventral) et
ectoblastiques. Les deux cornes neurales s'effilent et se
terminent dans le matériel caudal.
249
coloration neurale latérale dans les régions moyenne et postésornite et des régions voisinus
rieure du tronc en arrière du XI
de l'ectoblaste et du mésoblaste.
y) Ectoblaste.
FIG.17. — Maroom en haut de la zone marginale latérale. P1ntïiéS:
coloration en surface.; hachures : Coloration en profondeur. Cf. texte
Les limites externes des territoires cérébraux se sitt:ent
donc en arrière de la marque 156 et ont été touchées pur la
marque 157, ce qui nous a permis de les dessiner sur la
ligure 9 .
Les deux marques précédentes (156, 157, fig. 16 et 17)
empi:;tent toutes deux à la fois sur les limites des territoires
neuraux et ectoblastiques. D'autres expériences analogue
confirment qu'en . avant du territoire cérébral existe une
zone ectoblastique de la tète, que tout le long des cornes
médullaires latérales se trouve du l'ectoblaste. troneul.
L'époque tardive du revétemeut de la gouttière médullaire
243
•
.
Although it is easy to stain part of the brain anlage exclusively
by applying the Nile blue a little behind the centre of the disk, it
is almost impossible to make stain applied marginally and medially
affect the neural tube without affecting the brain. This indicates
that the neural zone at this level is quite thin but nevertheless, it
contains material which will eventually extend to the posterior
extremity of the embryo. This neural material, all of which is destined
to form the floor of the spinal cord, extends on both sides into two
neural horns, from which develops the lateral parts of the neural
plate. These horns were stained in embryos 98 and 99 (figure 14 and
15); stain 98 also affected the notochordal horn. This shows that the
thin tip of the anlage of the nervous system extends beyond the apex
of the notochordal horn ventrally.
As explained above, it is not possible to define exactly where
the ventral boundary of these horns lies. During the envelopment of
the yolk, marks applied to this part of the disk become so stretched
out that they are barely perceptible. Moreover, as the ventral parts
of the enveloping border do not invaginate until extremely late, the
blue stain becomes gradually paler and fades away. As we shall see
further on, the anterior parts of the disk can be considered to be
purely mesoblastic (ventral mesoblast) and ectoblastic. The two
neural horns taper off and terminate in the caudal material.
(iii) Ectoblast
The last two stains (156 and 157; figures 16 and 17) both encroach
upon the neural and ectOblastic territories. Other similar experiments
confirm that in front of the cerebral anlage there exists an area of
head ectoblast and that there is truncal ectoblast along the length of
the lateral neural horns. Since the neural groove is not covered by the
ectoblast until very late and the ectoblast is extremely thin, it is
almost impossible to demarcate the boundary between embryonic ectOderm
and extraembryonic ectoderm; the dividing line in Figure 9 merely
indicates where it is thought to lie.
(iv) Mesoblast
Some of the experiments described above also provide information on
mesoblastic development, Stain 103 (Figure 12) shows that the first
somites develop from material in the marginal zone right beside the
prochordal plate territory and the central part of the notochordal
territory. Since stain applied in this area is not incorporated into
the lateral mesoblast, the material from which this forhis must lie much
further out, It should also be noted that a stain applied at a given
level always appears in a fairly large number of myotomes. Furthermore,
when marks are applied to the lateral marginal zone, the bands of stain
in the embryo are always arranged very obliquely (see figures 12, 14, 15).
By way of example, in embryo 98 (Figure 14), the same initial stain
was found to affect the anterior part of the lateral mesoblast, the
in their
and 7, the following somites
outsidegfomts6
entirety, the inside edges of somites 18, 19 and 20 and the posterior
part of the notochord.
Thus, a stain applied to one particular area came to lie in the
lateral mesoblast anteriorly, the outside edges of the anterocentral
somites, the inside edges of the posterecentral somites, and the .
posterior notochord,
A similar pattern is evident throughout the lateral marginal zone.
The more the territories correspond to the median area of the bodu_tbe.
çloser they are to the median line on the surface of the lilastula..
We might note that the data Kopsch (1904) obtained with his
electrolytically produced lesions gave some indication of this strange
distribution of anlagen. A lesion produced at a given point on the
enveloping border would successively affect, from frortto back, the
myotomes, the intermediate mass, the renal anlage and the lateral
mesoblast.
It was observations such as these that led me to delimit the
prospective somites obliquely, having them slant outward and backward.
Below are described some supplementary experiments which, together with
those discussed earlier, make it possible to define the arrangement of
the somites more precisely.
See photocopy of original
Figure 18 - Stain in the posterolateral marginal zone. Dots: stain
on the surface; crosses: stain in the vitelline syncytium; bars: stain
in the chordamesoblast.
•
Embryo 140 (Figure 18) - At sixty-six hours, a long strip of stain
was applied along the edge of the disk, extending between 30 0 and 70 0
from the middorsal point. At ninety-eight hours the stained cells had
converged upon the midline, where they were actively invaginating. By
113 hours there was a strip of stain along the side of the embryo in
both the superficial and the deep layers. At 139 hours there was stain
•
•
J . PASTEELS
GASTRULATION DES VERT.EBRÉS.MÉROBLASTIQUES
par l'ectoblaste et la minceur de celui-ci rendent la démarcation entre ectoderme embryonnaire et extraernbryonnaire.
quasi impossible, et la limite que nous en avons tracée sur
la figure 9 est purement conventionnelle.
prévoir cette curieuse répartition des ébauches : une lésion
faite à un niveau donné dd bord d'enveloppernent,peu après
Son apparition, retentit spccessivernent en allant d'avant en
arrière, sur les myotomes, la masse intermédiaire, l'ébauche
néphritique, les lames latérales.
•
Ce sont de telles observations qui nous ont amené délimiter les . -somités présomptifs de façon oblique, s'étirant
fortement de dedans en dehors et d'arrière en avant.
Voici quelques expériences complémentaires qui permettent,
avec celles qui sont relatées plus haut, d'apporter des
précisions dans la répartition des somites.
.44
S) Mésoblaste.
•
.
•
•
•
•
•
Quelques-unes des expériences décrites plus haut nous
fournissent déjà des indications préciseS au sujet des
localisations mésoblastiques. Les premiers somites se trouveront d'après la marque 103 ( fi g. 12) dans la zone marginale,
—tout à côté des territoires de la plaque préehordale et de
la partie centrale de la chorde. Une marque faite à ce niveau
ne touche, d'autre part, pas encore les lames latérales. Le
matériel de celles-ci doit être reporté beaucoup plus en
dehors. Il faut aussi noter qu'une tache colorée, faite à . un
niveau donné retentit toiljours sur un assez grand nombre
de rnyotornes. Une autre caractéristique de ces résultats
obtenus par coloration de la zone marginale latérale, c'est
la disposition très oblique que prend la coloration dans le
corps embryonnaire (voir figures 12, 14, 15).
L'embryon 93 (fig. 14) montre, par exemple, pour une
même marque colorée, une coloration de la partie antérieure
des lames latérales, de la partie externe des somites VI-VII,
de la totalité des somites suivants, de la partie interne des
somites XVIII, XI X, XX et de la partie postérieure de la
, chorde.
Une marque faite à un niveau donné colore par conséquent
de la lame latérale antérieure, des parties externes des sornics
antéro-moyens, des parties internes des sornites postéromoyens et de la chorde postérieure.
Une disposition analogue se retrouve dans toute la zone
marginale latérale. Plus les territoires correspondent £i tics
parties médianes dans le corps, plus ils sont proches d... la
1.:711e ti la surface de la blastula.
ligne
Norons que certains résultats obtenus par KoPscli (100-1) à
l'aide de ses marques électrolytiques pouvaient déjà Faire
93h
•
/
/
1 \Ç
\
I.
\Zr'
:
\-7
i
245 -
,
-
•
.
/I \
,
/ N,.
i
i
;i 4 i'•;
I
FiR •
•
i;
FIG. IS. — Marque dans la zone marginale pg..stro-latérale.
• tillés ;coloration en surfaces ; croix : marque sur k syncytium vik flu;
ha...hures : colorati(:n du chrgdo-ntés ,Jbl.Kte.
Emb ryon 140 (fi g. 18) .
: marque allongée le long de la
marge du disque, s'étendant entre 30 et 700 du point ni édio-dorai
OS IL les cellules colorée: ont con...ergé jusqu'à la ligne médiane
où elles s'invaginent activement. 113 : la marque, tant dan.; le
plan soperfiert:1 que dans le plan profond, s'est rangée le long
>
See photocopy of original
Figure 19 - Posterolateral stain. Dots: Stain on the surface; bars:
stain in the deep layers; crosses: stain in the vitelline syncytium.
See text.
e
.1.■.
■■■
/
•
J. PASTEELS
246
GASTRULATION DES VERTÉBRES . :■ ÇÉROBLASTIQUner 2.17 ,
du corps embryonnaire. 139 h. : coloration cies VII, VIII, IX et
Xe somites et de l'extrémité interne des deux suivants; la partie
latérale du système nerveux est teintée au mérite niveau et un peu
en arrière. La chorde devait probablement être atteinte quoique
cela n'ait Pas été observé de façon certaine sur le vivant..
Embryon 112 (fig. 19). — 72 h. : marque repérée par la coloration du syncytiuni vitellin, s'étendant entre 120 et 155" du
point médio-cforsal. 90 h. : 1z colorzition s'est étendue dans la
partie postéro-latérale du plafond de la cavité'sous-germinale ;
elle se rapproche vers le côté clorsal comme le montre le repère
du syncytium vitellin. 113 h. : coloration sur deux plans du bord
d'enveloppement à 90 0 environ de la lèvre dorsale du hlai, topore.
137 h. : toute la partie postérieure et latérale gauche du bord
d'enveloppement est colorée. 1-16 h. : la coloration, toujours sur
deux plans, commence à s'intégrer dans le corps embryonnaire.
153 h. : sont colorés territoires sornitiques à partir du XX111':,
la lame latérale situéc quelque • peu en avant et l'ectoblaste
correspondant. L'embryon n'ayant pu survivre après ce stade,
on n'a pas pu déceler en quelle mesure le système nerveux était
touché par 121 coloration.
,---= \
=,.
,.,-----,
i
i
113h.
_I
1
(
' )
L'ensemble des repères siirs, (l'observation sur le vivant
n'étant pas toujours aisée) a été marqué le long des bords
interne et externe de l'ensemble cies territoires sornitiques
présomptifs, ainsi qiie le montre la figure 20. C'est à partir
de ces données que la démarcation des différents soinites
telle que nous la trouvons sur le plan de la Figure 9 n été
calculée.
Quant aux lames latérales, les observations no 144 et 9S
(fig. 13, 14) nous ont montré que leur extrén -iite antérieure
con-imenee au niveau du Ville somite présomptif. Plus
latéralement et ventrale -nient, le long du pourtour du disque
on trouve toujours une mince zone mésoblastiqtie latérale
bordant celle des mvotomes. Ce mésoblaste latéral se continue
vers la zone marginale ventrale, où, comme nous le verrons,
C« • I t ,;
'.• \ . . .;
'«> 1
k
P
I
en •—
b
•-- 7. _.,-,
p---7-.7-> P E3 .-_
C-1 ::!:,---
)
i
h.
le matériel qui s'inv4"..,Tine semble être uniquement cornpoté
de mésoblaste ventral, destiné ä la vésiéule
14r
Comme nous l'avons déjà dit, l'étude de ces matériau \
ventraux rencontre cependant des difficultés presquc insurmontables. On ne peut songer ni à conserver jusqu'I'l la
'r
: coloration en surf,c.
FIG. 19. — .\\arqué postéro-latéralé. Pointillé
du synytli.un
:
,:oloration
Croix
profondcur.
Hachures : coloratiun en
vitellin. Cf. texte.
,
24/
•
in sonlites 1, 8, 9 and 10 and on the inside edges of somites 11 and
12; the lateral part of the nervous system was stained at the same
level and posteriorly to a slight extent. The notochord was probably
affected although this could not be established with certainty from
observation of the living blastula.
Embryo 112 (Figure 19) - Stain was applied at seventy-two hours.
It coloured the vitelline syncytium and extended between 120 0 and
165 ° from the middorsal point. At ninety hours the stain had spread
into the posterolateral part of the ceiling of the subgerminal cavity
and was moving dorsally, as could be seen by gauging the distance
from the reference mark on the vitelline syncytium. At 113 hours there
was stain in both planes of the enveloping border about 90 0 from the
dorsal lip of the blastopore. By 137 hours the entire left posterior
lateral part of the enveloping border was stained. At 146 hours the
stain, still in both layers, was becoming incorporated into the embryo.
At 153 hours there was stain in the somitic territory starting in
somite 23, in the lateral mesoblast slightly anterior to this area and
in the corresponding ectoblast. Since the embryo did not survive
beyond this stage it was not possible to detetmine to what extent the
stain was incorporated into the nervous system.
All the firmly established reference points(since observation of
living embryos is not always easy) have been indicated along the
inside and outside borders of the entire prospective somitic area in
Figure 20. It was on the basis of these data that the somites were
demarcated as in Figure. 9.
Observations on embryos 144 and 98 (figures 13 and 14) -showed
that the anterior boundary of the lateral mesoblast lies.at the level
of the eighth prospective somite. Further laterally and ventrally,
around the periphery of the disk, there is always a narrow area of
lateral mesoblast bordering on the mesoblast of the myotomes. This
lateral mesoblast continues toward the ventral marginal zone where,
as we shall see, the material that invaginates seems to be composed
soley of ventral mesoblast destined to form part of the yolk sac.
As stated above, the study of the ventral material is fraught
with almost insurmountable difficulties. There is no possibility
either of preserving stain in a ventral territory until closure of
248
the blastopore or of staining the enveloping border directly at an
advanced stage. The most that can be done is to apply stain at the
posterior extremity of an embryo with fifteen or so somites.
See photocopy of original
Figure 20 - The firmly established reference points on the basis of
which the various somitic territories were delimited.
Embryo 180 (Figure 21) was stained a little to the left of the
posterior extremity (Figure 21a). Sixteen hours later (h) the stain
had become incorporated into both the somites and the nervous
system from the level of somite 21 to the hind part of the embryo.
Twenty-four hours after application of the stain the blastopore was
punctiform and the stain extended to the posterior tip of the
embryonic axis in both the neural layer and the mesoblast. The next
day the blastopore had closed and in its place was a ring of mesoblast
•
•
• 24g
•
•
fermeture du blastopore la coloration d'un territoire ventral,
ni à colorer directement le bord d'enveloppement à un stade
avancé. Tout ce que l'on peut faire, c'est placer des marques
colorées à côté dc l'extrémité postérieure d'un embryon
. d'une quinzaine de somites par exemple.
•
GASTRULATION DES VERTÉBRÉS MÉROBLASTIQUES
J. PASTEELS
:-
...blastique entourant un bourgeon caudal saillant. La coloration
•:se poursuit depuis le XX1‘.'e somite jusqu'au petit bourgeon
caudal, lequel est fortement coloré..
-
-1 (
Li
11
:
;1
t'S
es•
‘i•
de délimiter :cs
FtG. 20. — Sdkina des repères sûrs qui Ont permis
territoires des différents somites.
de
L'embryon ISO (fig. 21) a ét i . coloré un peu à gaticbc
marque
la
(b)
plus
tard
l'extrémité postérieure (5.2_ 21a). 16 h.
jusqu'en arrière de l'emse retrouve it partir dtt XXV sonnte
système nerveux. 24 h.
k
que
dans
sbmites
bryon tant dans les
la marque ••
punctiforme,
après la coloration, le blastopore est
tant
dans le
•
embryormaire,
s' étend jusqu'en arrière de l'axe
le
mésoblaste.
Le
jour
suivant,
le blasclanos
feuillet neural, que
A sa place, nous trouvons une aréole inéetopore est fermé.
2L —
Marque à la réunion du noeud postrieur et du bord
d'eoveloppement d'un embryon à 14 somites. Pointillés : colorati,m
en surface ou dans k s ysteme nerveux. llanures : coloration du
mésoblaste.
Comme le montrent d'autres expériences analogues, toute
,; marque atteignant les territoires situés après le XXIV,
s'étend toujours jusqu'en arrière de l'embryon et somite
colore tous les territoires — i part la chorde —situés après
' ,.ce niveau.
249
surrounding the small protruding the tail bud. The stain extended from
somite 14 to the tail bud, and was heavy in the latter area.
See photocopy of original
Figure 21 -- Stain at the junction of the posterior knot and the
enveloping border of an embryo with fourteen somites. Dots: Stain on
the surface or in the nervous system; bars: stain in the mesoblast.
As shown by other similar experiments, any stain. affecting
territories behind somite 14 always extends to the hind part of the
embryo and is incorporated into all the territories caudal to this
level, except for the notochordal anlage.
•
Although the mesoblast of the ventral part of the enveloping
border cannot be studied by means of vital stains, in vivo observation
and serial sections clearly show what becomes of it. This mesoblast
does not join with the embryonic axis. It reflects along the lips of
the blastopore as the latter becomes progressively smaller and after
the blastopore closes, it forms a ring around the tail bud which later
spreads out on the posterior part of the vitelline surface. As early
as 1904, Kopsch, in his study involving the localized electrolytic
destruction of tissuetstressed that there were no axial territories
in the ventral part of the disk.
(c) Comparison with other forms
The similarities between the pattern of'anlagen in the trout and
that in oligolecithal anamniotes are truly remarkable. As in
cyclostomes and amphibians, the major organ-forming territories are
arranged horizontally, in crescents. The neural and notochordal
crescents are superimposed in the dorsal half of the disk while the
mesoblastic crescent occupies the marginal zone on either side of the
notochordal region. In the trout, as in amphibians and cyclostomes,
notochordal and neural territories which from the median region of
the embryo from-the head to the posterior extremity of the trunk exist
in the middorsal region from the beginning of development. This . is of
fundamental importance, as will be stressed in the next section.
Thus, we see that partial segmentation in an anamniote has flot
change in the distribtuion of the anlagen.
realypoducngreat
With respect to the position of the endoblast, however, some
differences have been introduced and these should be pointed out. In
oligolecithal anamniotes the endoblastic crests are involved in the
formation of the roof and lateral walls of the archenteron, while
the large vitelline mass forms the floor of the archenteron. In
urodelean amphibians the entire truncal endoblast is subblastoporal;
the lips of the blastopore lie on the boundary between the endoblast
and the mesoblast. In Anurae and cyclostomes the endoblastic crests
are part of the marginal zone (see Vogt, 1929 and Weissenberg, 1934).
The invagination groove appears between the endoblastic material of
the lateral walls and that of the floor of the archenteron. On the
blastula, the marginal material of the endoblastic crests covers the
material of the lateral mesoblast, which is thus pushed back into the
internal marginal zone. In this regard, the arrangement of the
prospective endoblast in the trout differs even more from the urodelean
pattern. The entire endoblast Including the floor of the digestive
tube, in suprablastoporal in the trout. This material is superficial,
and seems to be extremely thin; it probably covers the mesoblastic
anlagen to a much greater extent than in cyclostomes and, Anurae,
although this last point is difficult to verify.
A similar arrangement is found in selacians (Van De Broek, 1935).
The pattern of anlagen in Scyllium closely resembles that in the trout,
at least on the dorsal side. The ventral side is characterized in
Scyllium as in Petromyzon (Weissenberg, 1934) by an interruption of
the marginal ring of chordamesoblast.
In the suprablastoporal position of the endoblast, then, the
trout is closer to the cyclostomes, selacians and. Anurae than to Urodelae.
But in the last analysis, it is to the anurean pattern that the
trout embryo shows. the greatest similarity. In teleosts as in Anurae,
the anterior border of the nervous system is below the animal pole;
the notochord and the nervous - system continue ventrally for a
considerable distancej and the somites are arranged obliquely and
continue ventrally for some distance.
252
4 - Morphogenetic Movements
The experimental results described up to this point, which have
served to establish the arrangement of anlagen on the surface of the
young blastodisk, show without any ambiguity that the morphogenetic
movements of the trout do not differ in principle from those of the
• amphibian. The cell movements that Vogt classified as invagination,
extension, convergence, epiboly and ventral divergence are easy to see
in the teleost. 1 propose to analyse in detail to what extent they
may be altered by the vitelline mass and to see whether they could
explain some of the structural characteristics of the teleostean
gastrula. The microscopie aspect, which has previously been investigated
in some excellent studies (see Hcnneguy, Goronowitsch, Sumner and so
on) will not be considered except in so far as it is affected by
the cell movements.
(a) Invag.41
endoblast
•
1.x.-PiA1 ILI.1
.211(3.1-n of
the
As explained above, the authors who have previously studied
gastrulation in teleosts have proposed a wide variety of theories
on the development of the deep layers. Goette (1873), Kowalewski "
(1886) and Chevey (1925) recognize that true invagination exists,
but for Henneugy (1888) it is complicated by cellular proliferation
and for Wilson (1889) and Boeke (1903), by delamination. Most authors
(Oellacher, 1873; His, 1878; Hoffmann, 1881; Ryder, 1884; Samassa,
1896 and Boeke, 1907) feel that delamination and "gastrulean cleavage",
are sufficient to iay ddwn 1116 - choreamesoblast and the endoblast.
The examination of coloured marks leaves no doubt. The entire
marginal zone pulls back little by little and rolls up along the
rim of the disk. Figures 11 and 19 and many observations of similar
•
253
.blastodisks show that there is true invagination. One may wonder,
however, whether delamination is not involved as a secondary
process, preceding and facilitating the invagination of the
superficial cells. Such "gastrulean cleavage" has been demonstrated
in anurean amphibians. The cells of the internal marginal zone become
detached from the superficial layer and tend to move up toward the
animal pole before the blastoporal line of invagination appears.
Although delamination does not play as important a role as Brachet
(1902) maintained, it is nonetheless true, as Vogt (1929) recognized,
that in anurae, gastrulean cleavage prepares the way for invagination.
•
•
It is conceivable that the marginal zone of the trout, which is
unusually thick, especially in eggs with the blastodisk protruding
above a large subgerminal cavity, might behave in the same way.
Examination of sections shows that this is not so. Figure 22a shows a
sagittal_ .section of a very young gastrula. The subgerminal cavity
is. well developed, the disk protrudes markedly and is not spread
out. The dorsal marginal zone is very thick. The drawing shows that
a mass of cells has streamed beneath the superficial layer, taking
a triangular form with a narrow apex anteriorly. A sharp dividing
line separates the invaginated cellsfrom the superficial layer.
There is no doubt that true invagination has occurred; the deep
cellular mass has a characteristic texture less dense, less crowded,
than the surface tissue. The cells are clearly engaged in active
movement and roll over each other, so to speak. The characteristic
texture is observed throughout the invaginated layer, right up to the
254
f.
line of reflection from the surface layer. It is clear that all the
cells begin to invaginate from the said line of reflection. The
sharply defined space between the two layers, which exists at all
stages, excludes any possible involvement of cleavage in a mass of
cells previously existing as one entity. The point where invagination
begins, the line of reflection, merits some attention. As invagination
is about to occur, the thick marginal zone suddenly becomes thinner
so that it can roll up as a unit into the deep layer. Figure 22b shows
Sec photocopy of original
Figure 22 - Two sagittal sections of young gastrulas that developed
from eggs with well developed subgerminal cavities. The gastrula in
(b) is four hours older than the one in (a).
a later blastula in which a similar thinning-out process has occurred
at the same place. Here again, the thick marginal zone of a protruberant
disk seems to have become flatter, to have stretched out, in order to
reflect back along its entire edge.
•
•
If the blastodisk is already lying flat on the surface of the yolk,
as occurs in S Fario and sometimes in S iridaeus, invagination is
simpler still-. Figure na shows an embryo of seventy-one hours. The
disk lies flat on the yolk and ventrally there are intercellular spaces
rather than a subgerminal cavity as such A few cells are becoming
detached from the dense dorsal mass, the heralds of invagination. A
tivement unique. Le point de départ de l'invagination, la
« charnière » 'doit attirer notre attention. Remarquons qu'a
ce moment la zone marginale épaisse s'amincit brusquement
de façon à pouvoir s'enrouler intégralement dans le feuillet
_55
GASTRULA -J- 10N DES VERTÉBRÉS .1É..ROBL.ASTIQUE
PASTEELS
•
, :
•
•
t;
.. •
émissaires de l'invagination. Une coupe sagittale d'Un œuf
de la mémé ponte, fixé 9 h. plus tard (fig. 6,b) nous présente
un amincissement de la zone marginale analogue au cas .
précédent mais de proportions moindres.
Ces mouvements d'invagination débutent au point
média-dorsal du disque. Peu à peu cependant, ils s'étendent
sur les côtés. Noué avons représenté sur la figure 23 des
reconstructions graphiques montrant l'étendue du feuillet
invaginé à quatre stades de la même ponte, différant de
3 en 3 heures. Notons que la limite externe du 'feuillet pro-
FIG. 22. — D:ux coupcs sagittales de jeune.s gastrulas provenant
d'une 1.14ante cavité sous-rminale très dz:velopp.3.e. b est 4 heures
phi› ;igé que a.
/Y.
if
P10 fond. La figure 22, b montre qu'un amincissement analogue
se produit. toujours au « même endroit à un stade plus avancé.
Ici encore, l'épaisse zone marginale d'un disque saillant
Semble se laminer, s'étirer pour se réfléchir par toute sa
tranche.
Si le blastodisque est déjà aplati à la surface du vitellus,
comme chez S. fario et parfois chez S. iridaetts, l'invagination
se fera d'une façon plus simple encore. La figure 6,a nous 2
montré un embryon . de 71 h. le disque est aplati sur le
vitellus, des espaces • intercellulaires repréSentent, du eût,:
ventral, une cavité sous-germinale qui reste virtuelle. Du
côté dorsal dense partent quelques cellules, premier: ,
\•1
11
tl!
i.1;
.
I.,
,,".„:•
.
,,
.
.,
N---- <.1-:-. --.'---- ---9
77n
\,.,.
---- 20 n
FIG. 23. — Quatre recrintructions graphiques montrant en hachures
le chordo-mésoblaste invai;iné. La ligne pointillée de la premiére
fig
ure indiqu2 les limites mal définie de la cavité sous-germinale.
fond est :.zéparée de la marge du ClisquC rar un usfmCe d'aut.u.nt
plus large que les cellules invaginées sont plus al-pondantes.
255
sagittal section of an egg from the same roe, fixed nine hours later
(Figure 6b), shows thinning of the marginal zone similar to that seen
in Figure 22 but not as marked.
These movements of invagination begin at the middorsal point of
the disk. Little by little they extend to the sides. Figure 23 depicts
the extent of the invaginated layer in eggs from the same roe at four
successive stages separated by intervals of three hours. Note that
the outside border of the deep layer is separated from the edge of the
See photocopy of original
Figure 23 - Four stages of invagination, with the shaded area
representing the invaginated chordamesoblast. The dotted line on the
first drawing indicates the poorly defined edge of the subgerminal
cavity.
disk by a space that becomes larger as the invaginated cells increase
in number. This space is actually the cleft produced by reflection
•
256
•
(figures 22b and 6b), which will of course be wider the larger the
mass of cells that has invaginated. It should be pointed out that
these drawings cannot give a true idea of the differences between
the actual numbers of invaginated cells in various areas. On the
midline there is a thick layer of cells, on the sides only a thin
tongue. However, the diagrams are adequate to show the progression
of invagination as the process extends to the entire periphery of
the disk, including the ventral side. Figure 23 shows this stage.
As in the preceding figures, the invaginated layer is represented
See photocopy of original
Figure 24 - Diagram of a more advanced stage showing an unbroken
ring of chordamesoblast. Bars: dense chorielmesoblast; dashes:
thinly distributed cells.
by bars. Here, the dashes indicate areas where the invaginated layers
consist not of a solid mass but or thinly - scattered cells or groups
of cells.
Observation of vital stains shows that the movements of invagination
continue throughout the period when the yolk is being incorporated Into
the embryo. The enveloping border thus constitutes a true blastopore,
homologous with the amphibian type. That is, the blastopore not only
surrounds the yolk so as to bring it inside the embryo but àlso invaginates
as a continuation of the chordamesoblast.
•
256
j. PASTEELS
Il s'agit là de la charnière de reploiement (cf.fig.22,b et 6,b)
qui sera évidemment d'autant plus épaisse que la masse
cellulaire en voie d'invagination sera pIùs abondante.
Remarquons aussi que ces reproductions graphiques ne
peuvent traduire suffisamment les différences qui existent
entré les quantités réelles de cellules invaginées aux divers
niveaux. Sur la ligne médiane, il s'agit d'une épaisse couche
cellulaire, sur les 'côtés cependant une mince languette.
Quoiqu'il en soit, nous y voyons la progression des processus
cl'inv:agination s'étendant à toute la périphérie du disque,
y-conipris le côté ventral. La figure 24 nous montrera un tel
•
GASTRULATION DES VERTÉBRÉS MÉROBLASTIOUES.7
Quelques protocoles nous serviront à illustrer cette •
évolution (1).
Embryon 117 (fig. 25). — 97 h. : gastrula à extrémité postérieure saillante. La marque qui est apposée sur ce « noeud terminal ). déborde plus fortement à gauche qu'à droite. 146 h. : la
Reconstruction graphique d'un stade plus avancé mouFIG.
rant un anneau continu de chordo-mésoblaste. Hachures pleines :
cbordo-mésoblaste massif ; hachures brisées : cellules éparses.
stade ; comme sur les figures précédentes, des hachures
représentent le Feuillet invaginé ; à l'intérieur de cette zoné
hachurée, nous avons tracé une région à hachures interrompues, plus mince, irrégulière, manquant , du côté dorsal ;
les Feuillets invagines représentés de cette façon né forment
plus une masse continue mais ne sont constitués que par
des cellules ou des groupes cellulaires épars.
L'observation des marques colorées démontre que ces
mouvements d'invagination se poursuivent durant - tout
renglobernent du vitellus. Le « bord d'enveloppement »
constitue donc un véritable blastopore, homologue du
blastopore d'un Àmphiblen c'est-à-dire un blastopore cp::
non seulement enfouit le vitellus à l'intérieur du germe, rn.:tiz;
encore invagine de façon continue du chordo-mésoblaste.
113h.
-
-- Marque colorée sur le nœud postérieur d'une jeune gastrula. Pointillés : coloration en sup.erficic ; hachures : coloration en profondeur. Cf. texte.
•
(1) Pour ces stades avancés, nous :Wons ,lia nous borner à faire dés
marques aux environs de l'extrémité postérieure de l'embryon, le
bord d'enveloppement étant trop mince et frag'ile.
257
A few accounts of experimental observations will illustrate this
processl.
Embryo 117 (Figure 25) - At ninety-seven hours, stain was applied
to a gastrula wich a protruding posterior extremity. The mark extended
further to the left of centre than to the right. At 146 hours the
See photocopy of original
Figure 25 - Stain applied to the posterior .knot of a young gastrula.
Dots: surface stain; bars: deep stain. See text.
'At advanced stages such as these, stain was applied only in the area
around the posterior extremity of the embryo; the enveloping border
was too thin and fragile.
L_)D
^
er the
Further
posterior part of neural plate was stained and in the deep
knotand the terminal notochord were heavily
heavier on the left, tapered off into the
posterior
which remained unstained.
forward the deep stain,
drawing
mesoblast on both sides of the notochord,
the
The experimental records indicate e^^t s^^iÿô^ ne.rvoustsy stem,
(no
po
the
was made) there was stain in the 1
the
terminal part of the notochord and
internal border of the myotomes.
right and was heaviest in the first ten somites.
to the
Embryo 119 (Figure 26) - At 121 hours stain was applied
neural plate, in the superficial layer
of an early
in the
At 143 hours, as stretching
posterior
laYer ,
only (stage intermediate between 2F and 2G).
extremity were two bands of stain in the deep
o
case,
there
above
from the posterior 'knot to the internal
sf
stbetch
r
sides of the unstained chopd11 t(^65somite 6).
as far forward as in embry
lied unilaterally
Embryo 160 (Figure 27) - A strip of stain was applied
an
of the disk, in the superficial layer only, of
e (intermediate between Figure 2G and 2H.)
the edge
embryo in the lanceolate stag
See photocopy of original
sterior extremity of an e-aYly
Figure 26 - Stain applied to the po
neural plate. Dots: stain on the surface and in the nervous
system; bars: deep stain. See text.
/
•
J. PASTEELS
partie postérieure de la plaque' médullaire est colorée ; e or . ofondeur, forte chloration du noeud postérieur, de la rx i. rtii:
terminale de la chorde; plus en avant la marque profonde s'effilti
dans le mésoblaste (coloration plus intense à gauche) de part
et d'autre d'une chorde incolore. 16 h. plus tard (le dessin n'a pas
été fait), les notes d'expériences indiquent une coloration de la
partie postérieure et gauche du système nerveux, de la partie
terminale de la chorde et de deux stries le long de l'extrémité
interne des myotornes. La strie gauche est plus marquée que la
droite et trouve son maximum clans les dix premiers somites.
,
GASTRULATION DES VERTÉBRÉS MEROBLASTIQUES
259
•
entre G et H, fig. 2). Le lendemain, la marque s'est concentrée
vers la ligne médiane et se trouve en partie invaginée dans le
.mésoblaste postérieur droit (fig. 27,b). Après 2 jours (fig. 27,c),
on retrouve une . partie de la marque dans le 1/3 postérieur
.depuis le XVII'? somite jusqu'en arrière. Après trois jours, tant
dans le feuillet superficiel que dans le feuillet profond, les terri- . toires colorés atteignent. le bourgeon caudal qui n'est cependant
que faiblement coloré.
...
EmUryon 119 (fig. 26). — 121 h. ; marque colorée bien iitnit,', e
- feuillet superficiel, sur l'extrémité postérieure d'une jeune au
rma,
— Marque colo r ée. sur l'extrémité postérieure d'une jeune
FIG.
pl.rque médullaire. Pointillés :.coloration en surface et dans le :-yst:'::ne
nerveux hachure : colonition en profondeur. Cf. texte.
plaque médullaire (stade intermédiaire entre F. et G de la fig. 2).
143 h. comme clans le cas précédent, on trouve deux stries
profondes partant du moud postérieur et touchant l'extrémité
interne des somites de. part et d'autre d'une chorde incolore.
165 Ii. : ces stries se prolongen t moins en avant que chez Ver:tin - on
117 (it la hauteur du VI ,' somite).
Embryon 1(30 (fig. 27). — Coloration unilatérale, assez allonzée
le long de la marge du disque, et atteignant uniquement 12
surlerficiel d'un 'embryon au stade dit lancéolé° (intertn,:diai':e
FIG. 27. — Marque colorée sur le noeud terminal et la zone avoisinante du bord d'envelopl.'erncnt d'une neurula. Pointillés : coloration
en surface et dans le systétne neritlx: hachures : coloration en profondeur. Cf. texte.
' Enfin, le résultat d'une marque à un stade plus tardif encore
nous a déjà été donné par l'embryon 1SO (fig. 21), Une marque
faite au niveau de l'extrémité postérieure d'un embryon à
XIV somites s'invagine encore et se retrouve dans le mésoblaste
depuis le XX(r somite jusqu'à l'extrémité du bout .geou caudal,
Une fois do plus, nous retrouvons do fortes analogies entr e
le développement des Amphibiens et celui des Téléostéens.
VOGT (1929) avait montré que l'extrémité postérieure de la
259
around the median line
The next day, the stain had concentrated
and had partly invaginated in the right posterior mesoblast (Figure
27b). After two days (Figure 27c) part of the stain could be seen
somite 17 to the posterior
in the posterior third of the embryo from
extremity. After three days the colouration, in both the deep and the
superficial layers, reached as far as the tail bud, which itself was
stained, albeit lightly.
See photocopy of original.
FJ:gure 27 — Stain in the terminal knot and around the enveloping border
of a neurula. Dots: stain on the surface and in the nervous system;
bars: stain in the deep layers. See text.
Embryo 180 (Figure 21) shows what happens to a stain applied at a
still later stage. Stain was applied to the posterior extremity of an
embryo with fourteen somites. The mark was found to invaginate and
appeared in the mesoblast from somite 20 to the end of the tail bud.
Once again, we find strong analogies between amphibian and
teleostean development. Vogt (1929) showed that the posterior tip of
•
260
the neural plate invaginates and Bytel (1931) was able to establish
that the invaginated material constitutes the posterior myotomes
of the trunk and the anterior myotomes of the taill, the notochord
excluded.
The only difference we can find between these results and our
own is quantitative. In the trout, the posterior extremity of the
neural plate invaginates and forms somites,but the posterior mesoblast
that invaginates at a late stage produces not only the posterior
part of the trunk and the tail but also the major part of the trunk.
At stage G the notochord has already invaginated. The blastodisk has
not yet incorporated even half the yolk. The blastopore is still very
large and as it closes, the mesoblast, which has already begun to
invaginate along the enveloping border, joins to the posterior
extremity of the embryo by a continuous converging movement. It then
finishes invaginating so that it lies on either side of the notochord
which tapers off progressively toward the back. It is the continuity
of these processes, which we will be discussed at greater length below,
that explains how the head and anterior part of the trunk can be well
constituted and differentiated while posteriorly, the trunk continues to
form through morphogenetic movements, not by a "growth centre" (see
below).
Another special feature of development in teleosts is the early
differentiation of a covering layer of cells (the "Deckschicht" of
th fGarman autbora),.1tIg_upwring layer is present as early as the
blastula stage in tn.e form of a continuous, somewhat flaftened
epithelium -at the edge of the disk in close contact with the vitelline
syncytium, with which it fuses, thus keeping the disk on the surface
1
•
According to Bytel, most of the tail myotomes of U.rodelae, and
the terminal myotomes of the tail in Anurae . develop from uninvaginated
material that arises from the neural plate. It was impossible to
study the formation of the tail bud in the trout because of
.
inadequacies of technique.
261
of the yolk 1 . Goette recognized that this layer did not invaginate.
Kowalewski (1886) pointed out a small bulge of cells on the posterior
border of the disk, the origin of which he could not ascertain but
from which he thought the endoblast derived. Boeke (1903) and Sumner
(1903, 1904) studied this "prostomial thickening" at the same time.
Both held it to be the source of the endoblast and Kuppfer's vesicle
but whereas Boeke felt this cellular layer to arise from the mitotic
activity of the vitelline syncytium, which would release endoblastic
cells on the superior border of the disk, Sumner thought that the
thickening resulted simply from reflection of the covering layer
downward. Lanzi (1909) attributed the origin of the pros tomial
thickening to the same process. Reis (1910) was of the same opinion
as Sumner.
Boeke's thesis is untenable because in muraenae, which he studied,
the vitelline syncytium is still in mitosis 2 when gastrulation begins.
In the trout, as in most teleosts, the syncytium at this stage is no
longer mitotically active.
The following observations were made on the basis of careful
examination of sections. At the very beginning of gastrulation, the
covering layer does not invaginate along with the posterior marginal
rim. However, the cells adhering to the vitelline syncytium are
affected by the movement in such a way that they crowd closer and
closer together and although initially flat, become cylindrical.
•
1
A few attempts to separate the blastodisk from the yolk have shown
that this can be done by splitting the covering layer away from the
vitelline syncytium.
2
3
The divisions are, however, multipolar and on the basis of current
data in cellular physiology it seems highly unlikely that a healthy
embryo could develop after haphazard distribution of chromosomes.
The release of a few cells from the vitelline syncytium in the blastula
cannot be ruledout. It is probably because of just such a process that
giant cells with lobulated nuclei are occasionally found in the deep
layers of the ask. These cells subsequently degenerate.
They become so crowded that the cells of the covering layer pile
up on top of one another, forming a cone. Thus, the "prostomial
thickening" in Figure 28a, which comes from the same embryo as the
section shown in 22a, is triangular in outline. In 28a there is
one cell that is attached to the prostomial thickening by only a
See photocopy of original
Figure 28 - (a) Details of invagination at the dorsal lip of the
blastopore, stage shown in Figure 22a; (h) Idem, stage of Figure 22b;
c) Details of a parasagittal section of a more advanced embryo.
thin stalk and which is obviously about to break off to join the
deepest layer of the invaginated mass. This cell is clearly fated
to become incorporated into the endoblast. The evidence is even
greater at a later stage (Figure 28b), where the continuity of the
•
•
•
26,
GASTRULATION DES VERTÉBRÉS MÉRnBLASTIQUES 263
J. PASTEELS
deviennent cylindriques. Ce tassement s'accentue au point
que les cellules de la couche de revêtement s'accumulent
les unes sur les autres et forment un amoncellement de coupe
triangulaire. C'est ainsi ,que se présente V« épaississement
prostomial » dc la figure 28,a qui provient du même embryon
que la coupe représentée en 22,a. On peut y voir une cellule
• • 0 / s..-
_., ::,•' ,.,..,....,—.:" -,-;,--.7" •--.; .., •
.,.• ,'—'—'.t.,—,:;;; /c_s fss-s , k...., .: ; ;
p. ,.,-,.--.--:;,.. f.,-:,....›...-,-....-,,i....-;< ,,, ,...c,:,
-,-,-"-, ,'-.c ...'..:-..-- ;.-.s.-, ,',t,,-- .
-
(
-•••••••
,
•
• • .•: :;
-
.
•—
•s
. .
•
/.; ':.-7Z*.•.)
•
'
P--.-•.se •
J
""" •
•
0•
;
•
"
..?
;
-
i;r7r-
•5
-
F;(.. as. -- a) Dtuil d'in‘- aginati ■ Al au niveau de la lèvre•drirsale
22.:. 1, ) Idem, stade do la fig. 22b.
du blast ,)pùr.:.: stade (.1. la
ej
coure parasagittale d'un embryrA pl us.
à peine rattachée à l'épaississement par un mince pédicule
et se trouvant manifestement sur le point de le quitter
pour gagner la couche tOà' à fait profonde du feuillet invaginé. Il est- évident Que la destinée de cette cellule est de
s'incorporer dans l'entoblaste. L'évidence est encore plus
grande pour un stade ultérieur (fig. 2S,b) ; la continuité de
l'entoblaste avec l'épaississement prostomial, de celui-ci
avec la couche de revêtement ne pourrait être niée. Nous
sommes en mesure donc de confirmer les observations de
SUMNER tout en Y ajoutant une explication dynamique de
cet « épaississement », dont l'origine pouvait sembler mystérieuse jusqu'ici. Toutefois, nous ne pouvons. comme
SUMNER le faisait, admettre une origine exclusive de l'entoblaste aux dépens de l'épaississement prostomial. En effet,
sur une coupe parasagittale d'un Stade plus avancé (fig. 28,c),
nous montrons un entoblaste parfaitement distinct, sans
aucune tendance à l'invagination de la couche enveloppante.
Nous pouvons donc admettre quel::: couche entoblastique
naît par invagination en même temps que le chordo-mésoblaste. L'entoblaste présomptif recouvrait primitivement
la zone marginale : après reploiement de celle-ci dans le
• feuillet profond, l'entoblaste se .trouvera sous le chordo'mésoblaste. L'entoblaste n'est formé par la couche de
revêtement que dans la mesure où celle-ci s'invagine, c'est-à.
dire uniquement aux environs de l'extrémité postérieure
• du disque.
La délamination n'intervient donc pas dans la constitution
des feuillets profonds :ceux-ci sont le résultat d'une véritable
invagination, c'est-à-dire d'un reploiement actif le long du
bord ma.rginal du disque. Certains auteurs comme GOETTE
(1S75) et SUMNER (1901) avaient, par le simple examen des
coupes, donné une interprétation correcte de ce mouvement
gastruléen. HENNECUY (188S) cependant, tout en admettant
l'in-vagination, la complique de prolifération cellulaire.
Bien souvent, ces phénomènes de prolifération localisée •
aux abords du blastopore ont été invoqués par les anciens
auteurs. Jamais cependant ces nuteurS n'ont apporté de
‘, éritables pieuve i l'nppui de leurs assertions. 11 est clair
que quelques mitoses, vues au hasard des coupe.s, ne peuvent
suffire ; ce qui importe. pour toute affirmation de ce genre.
c'est une étude statistique sur tOut un enlbrvon ou du moins
sur une grande partie d'embryon.
263
endoblast with the prostomial thickening and of the latter with the
covering layer cannot be denied. We are in a position, then, to
confirm the observations of Sumner, adding a dynamic explanation
of this "thickening", whose origin may have seemed mysterious up
to this point. However, we cannot allow, as Sumner did, that the
endoblast originates exclusively from the prostomial thickening.
In fact, on a parasagittal section of a more advanced stage
(Figure 28c) the endoblast appears quite distinct and the enveloping
layer shows no tendency to invaginate.
Thus, the endoblastic layer arises through invagination at
the same time as the chordamesoblast. The prospective endoblast
initially covers the marginal zone; after the latter has been
drawn down into the deep layer, the endoblast lies under the
chordlmesoblast. The endoblast is formed by the covering layer
only to the extent that the latter invaginates, that is, solely
around the posterior extremity of this disk.
Delamination is not involved in the formation of the deep
layers. Rather, they arise from true invagination, that is, active
reflection along the marginal border of the disk. Some authors,
such as Goette (1875) and Sumner (1904), arrived at a correct
interpretation of this gastrulean movement through examination of
sections alone. Henneguy (1888) however, while he recognized
that invagination occurs, brought in the complicating notion of
cellular proliferation.
These instances of localized proliferation around the blastopore
have often been cited by earlier authors but never has hard evidence
been offered in support of the assertions. Clearly a few mitoses
seen by chance on some of the sections prove nothing. What is
needed to substantiate a claim of this nature is a statistical study
of a whole embryo or at least the major part of one.
•
In recent years the concept of morphogenetic movements has
superseded that of "proliferation" to some extent. However
Wintrebert (1929, 1930) claims that there is in the anurean
amphibian Discogiossus "a stimulus to division propagated from
cell to cell", which arises in the centre of the gray crescent,
meets resistance at the vegetal pole and thus causes the blastoporal
lip to roll up. This author bases his statement on the behaviour
of stains applied dorsally. The present author cannot understand how
vital stains could have provided Wintrebert with information on the
mitotic rate of the cells. Similarly, Wetzel (1929 ), in the descriptive
introduction to his study on morphogenesis in the chicken, claims that
the thickening of the primitive streak is also due to cellular proliferation. Wetzel does, it is true, present micrographs of sections
showing numerous mitoses in this area. But once again, the appearance
of isolated sections cannot suffice to support a quantitative claim.
It is, moreover, somewhat strange that on the basis of the second
part of Wetzel's work (vital staining) one could very well interpret
the thickening of the early primitive streak as being due to axial
concentration.
•
Are both factors, morphogenetic movements and proliferation,
involved in morphogenesis? The possible role of cellular multiplication cannot be denied a priori. However mitotic foci, no
matter how abundant, cannot change the form of the embryo except
in terms of a localized increase in size. Mere division of cells
would be ineffective; the newly formed cells would also haveto
grow very rapidly. Such a-process would be particularly interesting
to observe in that it would imply a localized increase in metabolic
rate.
In order to consider this question objectively, I had recourse
to a statistical method. Camera lucida tracings were made of all
the sections of a hemiembryo, from the side to the median line,
•
265
and the exact position of all cells in mitosis was plotted on a set
of enlargements. Trout eggs lend themselves well to this method
since the cells are close together, with no spaces between them
except at the very beginning of invagination. One may object that the
same dividing cell may appear in several sections, but for technical
reasons it would have been almost impossible to eliminate this cause
of error. Although error of this nature would distort absolute
results, it can be ignored in relative measurements such as those
being made here. If the distribution of cells in mitosis was uniform,
the cause of error would be equally operative everywhere and nothing
would change. If, on the other hand, mitotic activity was not uniform
throughout the embryo, the error would be greatest where the number
of cells in division was greatest and in this way the differences
would only be accentuated.
In this work, the blastula stage was.examined. The disks could
be oriented by reference to the subgerminal cavity, which had already
formed. Comparison of a complete set of drawings of all possible
sagittal sections from the side of the embryo to the median line showed
that at this stage, the distribution of cells in mitosis is completely
uniform.
Uniformity was also observed in a second embryo, which had been
fixed as invagination was beginning. No localized cellular proliferation
could be observed either around the blastopore or in the invaginated
areas. The same findings were obtained from a third embryo, in which
the endochordomesoblastic layers where already well developedl.
Throughout the embryonic primordium the distribution of mitoses was
perfectly constant. We shall see further on that at this stage,
1
This embryo is shown in Figure 24.
the number of mitoses in the extraembryonic ectoblast falls abruptly
as the latter begins to thin out and surround the yolk.
The further objection may be raised that although the distribution
of mitoses is uniform, this tells us nothing about the mitotic rate.
The same number of cells may be undergoing division in all parts of
the embryo, but mitosis may occur more rapidly in some areas than in
others. In reply, one may point out that if this were the case, there
would be rapid changes in cell size. Even if the rapidly dividing
cells grew actively, it is unlikely that they could maintain the same
size as the cells in the other areas, and measurements made on considerably
enlarged camera lucida tracings have shown that all the cells remain the
same size.
We can therefore conclude that the formation of the deep layers
is due solely to cellular invagination, with no involvement of localized
cellular proliferation.
(b) Convergence
•
•
The question of convergence has been discussed even more than that
of invagination. No author has failed to remark that the teleost
embryo, which initially takes the form of a circular disk, soon becomes
drawn in toward one side and then actively lengthens toward the back,
while continuously assimilating the material of the enveloping border
on either side of the midline.
_
The interpretations to which these processes have given rise
are w11 known. The concrescence theory of His met with Singular
success and many attempts were made to extend it to other vertebrates.
Some particularly perspicacious authors did oppose these interpretations.
Two who were engaged in research in teleosts were Morgan (1895) and
Sumner (1904). Morgan conducted careful examinations of living
Crenilabrus eggs, and came to the following conclusions. The embryonic
267
•
•
•
primordium results from axial concentration, that is, from concentration
of material upon the median line combined with posterior movement.
The primordium thus formed lengthens as a unit and as it does so the
embryo incorporates material of the enveloping border along its sides.
These observations were confirmed experimentally. After unilateral
section of the enveloping border, Morgan did not obtain a hemiembryo but
rather defects in the lateral parts of the trunk. The growth of the
axial areas was not affected.
Sumner (1904), in addition to experiments whose.interpretation remains
difficult and which, in essence, stress only the great importance of
the terminal node published an excellent descriptive study based on
examination of several species (Salvelinus, Noturus, Schilbeodes),
to which reference has already been made in the discussion of invagination
and the formation of the endoblast. Sumner arrived at the same conclusions
as Morgan about the formation of the embryo:
it results from axial
concentration alone, with no true concrescence. However correct his
conclusions may have been, they only struck the first blow at the
concrescence theory. Kopsch (1904) inflicted lesions on the enveloping
border and obtained results similar to those of Morgan (1895). The
entire theory of His, according to which both sides of the enveloping
border fused with the median line, from one end of the embryo to the
other, was thus disproved by the facts. As explained above, Kopsch
altered the theory, introducing the notion of truncal growth centres,
which . he believed to be initially situated on either side of a cephalic
centre and to draw together by concrescence behind the latter, thus
forming the - posterior node
The posterior node would itself lengthen
to form the axial part of the trunk, while the lateral parts of the trunk
would develop from the enveloping border. Kopsch based this new model
of the concrescence theory on experimental results. Moser (1907) was
268
the only author to subject Kopsh's work to the severe criticism it
deserved, and this is to his credit. Apart from this single discordant
note, Kopsch's ideas were generally accepted. It was not until Vogt
(1926, 1929) failed to find any evidence of concrescence in amphibians
that further study of gastrulation appeared necessary.
It is evident merely from examination of the arrangement of the
anlagen in Figure 9 that Kopsch's thesis has no basis in reality.
In the centre of the blastodisk and in the posterior part of it there
exist organ-forming territories corresponding to the axial part of the
trunk from anterior to posterior, and these territories are almost
certainly present from the beginning of development. This was discussed
at some length in the previous section, so that there is no need to dwell
on it here (see figures 11 and 12).
•
There is no concrescence - such a process occurs neither before nor
during the formation of the teleostean trunk. There is, however, an
amazing degree of convergence during this period. Let us look again
at Figure 16. At seventy-seven hours, the stain was situated entirely
in the left anterior quadrant of the blastula; at 120 hours, it had
moved into the left posterior quadrant and at 166 hours it had become
entirely incorporated into the embryo. The stain of Figure 20, which
originally lay -even further- anteriorly, underwent even more typical
developmentl.
The movements of convergence are combined in a rather curious fashion
with movements of invagination. In Figure 19, for example, a stain applied
laterally to the blastula at sixty-six hours spread out along the edge
of the disk until its inside edge reached the median line. Since, as
we have seen, invagination begins in the middle and posterior parts of
the disk, it will be more intense here than on the sides. The interaction
0
It'should be noted that the ventrolateral territories undergo apparent
They begin by moving away from the median line but finish
divergence.
by converging toward the axis of the embryo.
•
269
of convergence and invagination produces two stained triangles, one
superficial and one deep, characteristically not directly in line with
each other. Figures 29a and b provide two similar examples; in 29c
the movements of the stained areas are analysed. Although invagination
is more intense in the centre
See photocopy of original
Figure 29 - (a and b) Two marginal stains undergoing the simultaneous
effects of invagination and convergence; (c) Diagram showing the direction
of movement (solid arrow: surface; broken arrow: deep layer).
than on the sides, convergence is more marked the further laterally one
looks. The movements of convergence begin in the superficial layer
(full arrows) and continue in the deep layer (broken arrows).
At subsequent stages it is principally the enveloping border that
undergoes movements of convergence, which occur both in the superficial
and in the deep layer.
•
•
These continuing process of convergence naturally bring the invaginated
material toward the midline and as a result there forms, immediately
anterior to the dorsal lip of the blastopore, in the posterior part of the
deep layer, - a cellular mass to which the attention of earlier authors was
many times drawn. Figure 30 shows a midsagittal section (b) with two
4.
263
.
.
-
. •
, .
PASTEELS
GASTRULATION DES VERTÉBRÉS MEI-cOBLASTIQUE.69
Un seul auteur, F. MOSER (1907) a eu le mérite d'en faire
une critique sévère, mais fuste. A part cette seule note
discordante, les idées de koPscIt ont été admises de façon
générale. Ce n'est qu'à la suite des travaux de VOGT (1926,
1929) qui ont montré l'inanité de la concrescence chez les
Amphibiens, qu'une nouvelle étude de la gastrulation a paru
•
nécessaire ..
Le seul examen du plan d'ébauches que nous publions
sur la figure 9 montre bien que la thèse de KOPSCH ne correspond à aucun fait réel. Sur la blastula, et sans nul doute
dès le début du développement, au centre et en arrière du
blastodisque existent des territoires qui correspondent à la
partie axiale du tronc depuis l'avant jusqu'en arrière. Nous
avons, dans le chapitre précédent, eu suffisamment l'occasion
de le souligner pour qu'il soit encore besoin d'y insister ici
(cf fig. 11 et 12).
Il n'y a donc pas de conci-escence, ni avant, ni pendant la
formation du tronc du Téléostéen. La convergence cependant .
l'interaction de la convergence et de cette invagination nous
vaut deux triangles colorés, l'un superficiel, l'autre profond,
décalés de façon caractéristique l'un par rapport à l'autre.
Les figures 29a et b en donnent 'encore deux exemples similaires; en 29c, nous figurons un diagramme analysant les
mouvements de ces territoires colorés ; si l'invagination est
FIG.'29.
yestd'unamplrxoie.Rtunsparxmle
à la figure 16. La marque était tout entière située dans le
quadrant gauche (,;t antérieur de la blastula (77 h.), à 120 h.
elle en occupe déjà le quadrant gatiche et postérieur,
à 165 h. elle est tout entière incorporée dans le corps embryonnaire. La marque de la figure 20, primitivement plus antérieure encore., présente une évolution encore plus typique (t).
Ces mouvements de convergence se combinent de façon
assez curieuse avec les mouvements d'invagination. Voyons
par exemple la figure 19. Une marque latérale sur la blastula
(66 h.) s'est allongée le long du bord du disque jusqu'à
atteindre par son extrémité interne la ligne médiane.
L'invagination, avant, comme nous l'avons vu précédemment, débuté au milieu et en arrière du disque, y sera
évidemment plus :intense que sur les côtés. Le résuft'at de
(t) H est à noter guc les territoires ventro-lateraux subissent nne
Ji vergen.:;: :typa/ cille : ils commenent par ›'é,.*.arter du mé-
(iane. Ils finissent cepen:lant par converger vers l'axe embryon:Il:1'2.
—
C.
•
a et b) Detrx marques marginales subissant les effet
simultanés rie l'inv2zinatir,n et de la converrence. c) Schéma rep
tant la direction du mouvement (1cbc pleine :sur(ace i1èch2 en tirets :
profondeur).
.
plus intense au milieu que sur les côtés, en revanche la
convergence sera d'autant plus forte qu'elle affecte des
territoires latéraux ; ces mouvements de convergence
débutent dans le feuillet superficiel (traits pleins) et se
poursuivent dans le feuillet profond (tirets).
A des stades ultérieurs, c'est surtout le bord d'enveloppement qui sera soumis aux mouvements de convergence,
ceux-ci se faisant sentir tant pour le ftuillet superficiel que
pour le feuillet profond.
Ces processus continus de convergence vont naturellement
accumuler le matériel invaginé vers la ligné médiane et vont
ainsi amaz:ser inimédiareme.nt en avant de la lèvre blas- .
toporale dorsale, c'est-à-dire dans la partie postérieure du
'feuillet profond, une masse cellulaire Sur laque.I12 l'attention
des anciens auteurs a été mainte:, fois attirée. La figure 30/i .
montre une coupe sagittale entourée cie part et d'autre de
il
270
•
sagittal sections (a and c) on either side. (The embryo was sliced into
195 10-micron sections; (a) represents section 103, (b) section 109, and
(c) section 115.) The diagrams reproduce only the contours of the layer
as they appeared in carefully executed camera lucida tracings. Note the
thickening of the posterior part of the deep layer; it constitutes a
See photocopy of original
Figure 30 - (a,b,c) Three sections: a midsagittal section between two
sagittal sections of an embryo at the stage of Figure lc; (d) Transverse
section through the anterior extremity of a slightly more advanced embryo.
•
true "cellular node". Figure 29e clearly shows how it originates. Later,
as the invaginated cells move forward, the entire deep layer shows a
midline thickening in the form of a long cord. Figure 30 shows the outline
of a transverse section passing toward the anterior extremity of a slightly
more advanced embryonic primordium; the anterior extremity of the axial
cord, the "Achsenstrang", as Goronowitsch (1885) called it, is apparent.
This "axial cord", which arises through convergence of deep elements,
constitutes the first primordium of the notochord. It will be discussed
further in the following section.
•
.
GASTRULATION DES VERTÉBRÉS .MÉROBLASTI•
deux coupes parasagittales a et c. (L'ensemble de l'embryon
comportait 195 coupes à 10 [.2., la coupe a est la 103.?, la coupe
b la 109c, la coupe c Ia 115P), Nous n'y reproduisons que les
contours des feuillets tels qu'ils ont *été dessinés soigneu-•
sentent à la chambre claire. Remarquons l'épaississement
de la partie postérieure du feuillet profond : il existe là un
•
C
O.
FIG. 30. — a, b. c) Trois coupes :une sagittale encadrée de deux parasagittales d'un embryon au stade de. la fig .. Ic. d) Coupe transversale
à travers l'extremitz' antérieure d'un embryon un peu plus avancé.
véritable « 11(2 Mi cellulaire ». Le diagramme de 1.a figure 29
en démontre l'origine de façon évidente. Plus tard, au fur
et . à mesure que les cellules in- vaginées progressent vers
l'avant, c'est tout le feuillet profond qui présentent un
épaississement médian en forme de Cordon allongé. La
figure. 30d représ- ente les contours d'une coupe transversale
passant vers l'extré.mité antérieure d'une ébauche embryonnaire un. peu plus avancée. Nous y voyons l'extrém:té
antérieure de ce- cordon axial, cet « Achsenstrang » comme
l'appelait GORONOWITSCH (1885),
Ce « cordon axial se résultat de la convergence des éléments
profonds, va constituer la première ébauche de la chorJe.
Nous la retrouverons 'clans Ic paragraphe suivant.
271
• Remarquons enfin que les processus de convergence sont
loin d'étre limités aux premières phases de la gastrulation'
proprement dite. L'examen des figures 1, 2 et 3 montre à
quel point les ébauches embryonnaires primitivement étalées,
élargies, se rétrécissent. Ces processus résultent d'un mélange
de convergence et d'extension comme nous le verrons un
peu plus - loin ; nous aurons l'occasion d'étudier en détail
ces mouvements de convergence tardive en examinant la
voûte archentérique. •
1.■•••
••••
•
c) Exterzsion
Comme chez les Amphibiens, les territoires axiaux
montrent dès le début de la gastrulation une tendance
extraordinaire à l'extension. Des marques colorées sur la
ligne médiane, que ce soit dans le matériel médullaire ou
dans le matériel chordal, s'étirent et s'étendent toujours
fortement vers l'arrière. La figure 11 en donne un bel
exemple.
Comme chez les Amphibiens aussi, c'est grâce à cette.
• extension des territoires axiaux que le territoire neural,
primitivement étalé en croissant, viendra s'étirer en un
tube antéro - postérieur. La combinaison de la convergence
des parties latérales et de l'extension des territoires axiaux
y déterminent un mouvement complexe que GOERTTLE:Z
(1925), utilisant un ternie militaire, a.qualifié de « Schwenkung ”. On le retrouve chez la Truite avec exactement Ie
mêmes modalités que chez les Amphibiens. Les figures 36,D
F en donnent une représentation schématique. Tandis
la partie centrale de l'é'oauch.e cérébrale reste fixe et ne
s'allonge que modérément, les parties latérales y convergent.
Les mouvements sont beaucoup plus étendus pour Le:
territoires médullaires : extension intense des territeir..:›
médians jusqu'en arrière dc l'embryon tandis que la convergence rabat de part et d'autre les matériaux latéraux
la ligne médiane. La figure 11 déjà cirée montre •I'e:s:ténsi( , n
des territoires médullaires rr,édians qui ne constiturc.::::
que la partie axial e de la plaque, c'est-:-dire le planci:er du
271
It should be noted that the process of convergence is far from
being limited to the first stages of gastrulation as such. Examination
of figures 1, 2 and 3 shows how much narrower the originally flat,
spread out embryonic primordia become. This results from a combination
of convergence and extension, as we shall see below. The late movements
of convergence will be considered in detail in the discussion of the
roof of the archenteron.
(c)
Extension
In the trout as in amphibians, the axial territories show an
extraordinary tendency to extend, right from the beginning of gastrulation.
Vital stains applied to the median line, whether in the medullary material
or the notochordal material, always become drawn out and extend markedly
toward the caudal end of the embryo. Figure 11 shows a good example.
Also as in amphibians, it is because of this extension of the axial
territories that the neural territory, originally crescent-shaped, becomes
transformed into a tube stretching anteroposteriorly. The combination of
convergence - of the lateral parts and extension of the axial territories
produces a complex movement that Goerttler (1925), using a military term,
dubbed "Schwenkung". It can be observed in the trout to occur exactly
the same way as in amphibians and is diagrammed in figures 36D to F.
While the central part of the cerebral anlage remains stationary and
lengthens only moderately, the lateral parts converge. The movements are
much more extensive in the neural territories; there is active extension
of the median territories right to the posterior, tip of the embryo while
convergence brings the lateral material on either side toward the median
line. Figure 11 shows the extension of the midline neural areas which
will constitute only the axial part of the plate, that is, the floor of
•
•
272
the neural tube. To see the convergence of the lateral parts of the
nervous system, the reader should examine figures 17, 18 and 19.
As we have seen, the notochordal territory undergoes a movement
of elongation even before invagination begins; this movement continues
and intensifies in the deeP layer and becomes one of the principal factors
in the formation of the body of the embryo. Figure 10 provides a good illustrati,
of this. The median stain applied to a blastula of seventy-one hours
had lengthened before invagination began (eighty-nine hours). At fourteen
hours, the deep part of the stain constituted a wide cephalic plate
(prochordal plate) behind which was a wide axial band. At 138 hours,
while the prochordal endoblast had changed little, the notochord had
become narrower and markedly longer.
Let it be noted in passing that there is remarkable parallelism
between the movements of the superficial layer (neural anlage) and those
of the deep layer (prochordal plate and notochord). Both display little
extension and convergence in the cephalic parts, and intense movement
in the truncal territories.
•
Examination of sections and diagrams brings remarkable confirmation
of these observations on the elongation of the notochord obtained through
study of the changing form of vital stains. Henneguy (1888) noted that
in the early stages, the notochordal anlage is very wide. Later, he
observed narrowing of the notochord combined with a diminished number
of nuclei in a transverse section and correctly concluded that the
originally wide anlage was elongating.
Figure 31 shows the notochordal anlage at four characteristic stages.
The first embryo, 123 hours old, displays a very early neural plate. The
notochord, which has only just separated from the mesoblast, is in the
shape of a very long truncated cone. The differences in thickness
anteriorly and posteriorly can be seen even better on the diagrams of the
sections made at the levels marked a, b and c. The reason for the
conical form is quite apparent: the processes of invagination and
•
273
convergence (see above, figures 29 and 30) are still more prominent than
the movements of extension; the notochordal material is accumulating
more and more ma posteroanterior direction. However, at 138 hours, the
situation had already changed. Although there was slight narrowing
anteriorly and dilatation posteriorly, the major part of the notochord
was cylindrical and not as wide as previously. By 145 hours, there was
marked narrowing in the anterior two thirds and the process was still
continuing, because not until 160 hours did the notochord attain its
final diameter in the anterior parts of the embryo, at the
See photocopy of original
•
Figure 31 - Four drawings of the dorsal notochord. Beneath the first
drawing are shown the outlines of the three transverse sections made
at the levels indicated by a, b and c.
•
a
^
GASTR[;LATIO:Y DES ^'ERTÉBRFS MÉROBLASTIQL'E' •.S
273
J. PASTEELS
tubé médullaire. Pour la convergence des parties latérales
du système nerveux, que l'on examine les figûres 17, 18 et 19.
Nous avons vu que le territoire chordal subit déjà un
mouvement d'élongation dès avant l'invagination ; ce
mouvement se poursuit et s'intensifie dans le feuillet profond
et devient dès lors un des facteurs principaux de la formation
du corps embryonnaire. La figure 10 en donne un bon exemple. La marque médiane appliquée sur une blastula de
71 h., s'est allongée déjà au début de l'invagination (89 h.).
A 14 h., la partie profonde de la marque constituait une
larg^ plaque céphalique (plaque' prechordale) suivie d'une
large bande axiale. A 138 h., tandis que l'entoblaste préchordal n'a guère changé, la chorde s'est rétrécie et allongée
à l'extréme.
Soulignons en passant le remarquable parallélisme qui
existe entre les mouvements du feuillet superficiel (ébauche
neurale) et du feuillet profond (plaque Préchordale et chorde):
peu d'extension et de convergence dans les parties céphaliques, mouvements intenses dans les ébauches troncales.
(voir plus haut, figures 29 et 30) dominent encore sur les
r
mouvements d'extension ; les matériaux chordaux s'accumulent de plus en plus d'arrière en avant. Mais à 138 h,.
la situation est déjà changée ; s'il existe encore un léger
rétrécissenlent antérieur et une dilatation postérieure, la
majeure partie de la chorde est cy lil;driquc, moins large que
dans le cas précédent. A 145 h., le rétrécissement est devenu
extrême dans les deux tiers antérieurs, il va pourtant se
poursuivre encore car ce n'est qu'à 160 h. que nous trouvons
le diamètre dé;initif de la chorde dans les parties antérieures
'zi
I
145,L
F:
Cette élongation de la chorde étudiée ainsi par les cllangerlents de forme des marques colorées est remarqu;:blement conirnl^e par l'étude des coupes et des reconstructions
g raphiques. HF`Nt:ct;Y (ISSS) a^•;:it déji, noté qu'aux jeunes
stac!es, I'éi^^.uehe chc^rdale était très élargie. Plus tard,
observant un rztr ci^;cment de la chorde allant d'ailleurs
de pair avec une d!iP.illiltton du nombre
d,1n^
,^^^
^--_= â\ \
une coupe tri;ns^er^;l1 , cet auteur en avait légitimement
co:?c-lu iI une élongation de l'ébauche primitivement large.
Sur la fioure 31, nous voyons les résultats iie recot.structio,ls grah?Iic!uc; dc !'éb;ui^t!e cltordale à quatre stade.i
car,ictcri,tiques. 1_e premier embryon de 123-k !l. montre
une toute jeune plaque nleLluil:zir^. La chordc s'est à peinC
séparée du rn
Elle a l;r forrnc d'un cône tronqué
très a!(vn;;^. Les difiércnces d'épaisseur en avant et cri
arrière ressortent erlcor^: mieux Je
Lies coupes faites
a U.K ni\'uaus, 0, v ut c. Celle forme aô!rique se comprend
aisément : les pru:e^s!!s d'itlv;:gination et de convergence
1^^^7Ji7
%IG. ji.
- Quar ::C!,u^t.'l:iTlJ^5 zrat•, ;.1I,:
' 1.
S d.-, l:: ih,it'^!^ t vt';a^,•.
Sous In ^^Ct'^1!a^, l^j i0nf
a, Let c..
ours d^ tC(l1a
P- trZnsv(:;',:li2sauslliv :iux
•
274
same time as the myotomes were forming there. Caudally, there was a
dilated area which terminated in the posterior node, merging into
material that was as yet undifferentiated. This series of drawings
shows that at the beginning of the formation of the body of the embryo,
the entire notochordal anlage elongates and extends. Later, when the
anterior parts are already in place, elongation continues at the expense
of the posterior dilatation, to which newly invaginated material will
gradually be added. At the 160-hour stage depicted in Figure 31, the
notochordal material has finished invaginating, as we have seen (p 260),
and the subsequent development of the notochord results purely from
extension of the posterior territories. Note the remarkable agreement
between the results given by the in vivo staining of a good part of the
notochordal territory (Figure 10, 138 hours) and the drawing of the
notochord of an embryo fixed at the same stage (Figure 31, 160 hours) 1 .
The territories that lie on the axis of the embryo from the beginning
of development are not the only ones to extend. The areas that are
initially lateral, after convergence has brought them into contact
with the body of the embryo, undergo extension which is less marked the
more lateral the initial position of the area, as can be seen from figures
15, 16 and 17.
(d) Structure and significance of the terminal node
Throughout gastrulation, the caudal end of the embryo displays a
swelling consisting of undifferentiated cells. This "terminal node"
has been regarded by most authors as the "growth centre" of the teleostean
trunk.
1
•
The difference in age between these two embryos at the same stage is
due to the eggs having been kept at different temperatures.
•
275
Kopsch (1904) thought it to be the truncal growth centre that resulted
from the fusion of two initially separate centres. Although Kopsch was
not explicit about the term "growth", many authors, among them
Oellacher (1872, 1873), Goronowitsch (1885) and Henneguy (1888), took it
to mean intense cell proliferation. This was also the way Brachet used
the term in the first edition of his Traité d'Embryologie (1922).
The author decided to determine the validity of this concept by
making precise counts, in the same way as was done for invagination.
Embryos at fairly advanced stages, the axial organs of which had already
differentiated anteriorly, were chosen for study. All the sections of
the terminal node and all the corresponding areas of the various organs
in the head and trunk were drawn to considerable enlargement. The areas
were measured with a planimeter, the volumes calculated, taking account of
the enlargement and the thickness of the sections, and cells in division
were counted. Results were as follows:
•
Volumes
in mm3
Number of
mitoses
0.243
87
Brain
0.263
126
Spinal cord
0.273
111
406
Myotomes
0.272
104
382
Lateral mesoblast
0.101
44
436
106
460
0
0
Posterior node -
-
Mitoses
per mm3
317
_
•
475
,
Posterior mesoblast
(still unsegmented)
0.230
Notochord
not calculated
One result stands out - there is no longer any mitosis occuring in
the notochord at this stage. The notochord is already well differentiated,
and it is well-known that the mitototic activity of the notochordal cells
stops early. The other values reveal that there are differences, sometimes
quite marked ones, between the various differentiating organs. Although
cell proliferation does not appear to play an appreciable role during
gastrulation, it becomes increasingly important in the later stages of organ
•
276
differentiation. Time was not sufficient to research this aspect far.
Suffice it to remark that in the posterior node the number of mitoses
per unit area is not higher than elsewhere, but lower. The posterior
node is not a centre of cell proliferation. In explaining its true
significance, we could best describe it as the crossroads of the
morphogenetic movements. It is the dorsal lip of the blastopore, where
invagination, convergence and extension all combine to accumulate material.
This material may remain here for quite a long period, as do the posterior
parts of the notochord, or it may be located here very transiently, as is
the mesoblast brought in by the enveloping border, which subsequently comes to
lie along the side of the embryo.
These data suffice to explain the unusual structure of the posterior
node, which has intrigued many embryologists. Henneguy (1888) described
the node in detail, without being able to offer a word of explanation.
It consists of a bud of undifferentiated cells, without layers, continuous
on the sides with the enveloping border. Anteriorly, as can be seen on
the longitudinal section of Figure 32a, it extends into the three layersthe superficial layer (neural plate), the middle layer (notochord and
mesoblast), and the inferior layer (Kupffer's vesicle and endoblast).
Figures 32b, c and d show three successive transverse sections of the same
embryo, passing-respectively through the posterior node the point where
the layers appear, and a little further forward than this. It can be seen
that the dells of the posterior node are arranged concentrically, like the
layers of an onion, and that the fissure separating the neural plate from
the notochord separates the "onion" into two unequal halves of symmetrical
structure. The reason for this arrangement is quite apparent from what
we already know about morphogenetic movements. In both the superficial
and the deep layers, the cells are compressed from above down by invagination,
•
277
and from
of cells
that the
the thin
outside to inside by convergence. The considerable accumulation
in this area explains what Sumner (1904) understood some time ago,
demarcation of the layers takes place further anteriorly than
enveloping border.
See photocopy of original
•
Figure 32 - (a) Sagittal section passing through the terminal node;
(b) Transverse section passing through the terminal node;
(c) Transverse section through the area where layers appear;
(d) Transverse section cut a little further forward.
(e) Epiboly and divergence
Ii
The Movement of epiboly, very markdd in teleosts, is so clear to see
that it should require little comment. But there is the question whether
•
6
•
•
J. PASTEELS
•
stades ultérieurs de différenciation des organes_ Nous
n'avons d'ailleurs pas eu le loisir de pousser cette étude dans
ce sens. Qu'il nous suffise de faire remarquer qu'au niveau
du noeud postérieur le taux des mitoses au lieu d'être plus
élevé qu'ailleurs, est en réalité plus bas. Le nœud postérieur
n'est pas un centre de prolifération cellulaire. Quelle est sa
signification réelle ? C'est un véritable carrefour de.tous les
mouvements morphogénétiques ; c'est la lèvre dorsale du
blastopore où invagination, convergence et extension concourent à accumuler des matériaux. Ceux-ci peuvent y
séTourner un temps assez long comme les parties postérieures
de la chorde. ; ou bien encore y passer de façon très transitoire
comme le mésoblaste qui y vient par le bord d'enveloppement pour s'appliquer ensuite le long du corps embryonnaire.
Ces données suffisent pour expliquer la structure très
particulière de ce noeud postérieur, structure qui jusqu'ici
avait intrigué de nombreux embryologistes. HENNEGUY
(188S) en particulier en a donné une description minutieuse
sans pouvoir y ajouter un mot d'explication. Ce noeud
consiste en un bourgeon de cellules indifférenciées, sans
.feuillets, se continuant suries côtés avec le bord d'enveloppement ; en avant, comme on peut le voir sur la coupe
longitudinale de la figure 32a, il se prolonge dans les 3 feuillets superficiel (plaque médullaire), moyen (chorde et
et entoblaste).
.
més.oblaste) et inférieur (vésicule de 1<upffer
montrent
trois
coupes
transversales
du
Les figures 32b, e, cl
M C."Ine embryon passant successivement a. travers le noeud
postérieur (b), à l'endroit où les feuillets apparaissent (c)
et un peu plus en avant (d). L'on verra que les cellules du
noeud postérieur sont disposées concentriquement, en bulbe
d'oignon et que la fente venant séparer. la plaque médullaire de la chorde sépare ce bulbe en deux moitiés inégales
niais à structure symétrique. Ce que nous savons des mouvements morphogénétiques il ce niveau nous explique
clairement cette disposition. Tant dans le feuillet superficiel que dans le feuillet profond, les cellules y sont coni-
GASTRULATION DES VERTÉBRÉS MÉROBLASTIQUES 21/:
•
primées de haut en bas par l'invagination, de dehors en
dedans par la convergence. Enfin, la .forte accumulation
cellulaire à ce niveau nous explique, ce que S. UMNER (190-l)
avait déjà compris, que la démarcation des feuillets s'y fasse
à un niveau plus antérieur que dans le mince bord d'enveloppement.
e
-
- .
•
.
Fia. 32.
a: Ct .•upe sazit tale p::,an t. par le ii euci terminal. C.:, d:
Trois ci;up:s rransv2rsal2s . pass.int h) à travers 12 ncuud c) an
nivcau uO ks uilkt e séenient. d) un p2u plus en avant.
—
e) Epibol'i£.' et divergewe
Le mouvement d'épibnlie, si intense chez les Téléos -téens,
est d'une évidence telle que nous n'aurions pas grand chose à
en dire. Mais ici aussi se pose la question de savoir si l'enve-
•
278
the envelopment of the ventral mass of yolk results from cell proliferation
or from cell movement. That there is movement is quite certain. One need
only compare the thick roof of the subgerminal cavity with the thin
unicellular layer that is left at the end of gastrulation. The cells
finish by changing their shape, spreading out thinly like an endothelium
(see Morgan, 1895).
To determine to what degree cell proliferation was involved, dividing
cells were counted in the embryogenic and extraembryonic areas of a
gastrula, at the stage where the roof of the subgerminal cavity was beginning
to become thinner (embryo of Figure 24). The distribution of dividing cells
had been found to be uniform throughout the embryonic area. Measurements of
the areas of the embryogenic and extraembryonic regions with a planimeter,
calculation of volumes and counting of cells in division gave the following
results:
•
Volume
in mm3
Number of
mitoses
Embryogenic area -
0.444
165
Extraembryonic area
0.354
91
Mitoses
per mm3
-
375
260
The difference was considerable, and it was found to increase as time
went on. Soon it was no longer possible to find a single dividing cell in
the ventral epiblast. Thus it can be seen that the process of epiboly
is not accompanied by cell proliferation.
Ventral divergence, a movement observed by Vogt in amphibians, consists
in reflection of the already invaginated ventral mesoblast along the sides
of the embryo, so that it moves away from the axial organs. In the trout,
this movement cannot be seeniuntil after the blastopore has closed, when
the ventral mesoblast that arises from the most anterior part of the
enveloping border is being laid down. This movement does not seem, then,
to be particularly prominent in teleosts.
•
•
279
(f)
Comparison with other forms
In their general outline, the morphogenetic movements of the trout
are remarkably similar to those of amphibians. The condensation of the
yolk into an inert mass, a considerable change, has little effect on the
essential events of gastrulation. The same morphogenetic movements are
combined in much the same way in totally segmented anamniotes whose vegetal
cells take part in morphogenesis and in anamniotes in which the major
part of the egg consists of a huge inert mass attached to the digestive
cavity. Thus, the profound alteration of these gastrulean movements
that we can expect, on the basis of Wetzel's research on gastrulation in
birds (1929), to encounter in amniotes will have to be attributed to
some reason other than the mere accumulation of yolk at the vegetal pole,
such as is seen in teleosts. Whether the marginal zone surrounds an
unsegmented mass or vitelline cells, the principle is the same. It should
be noted that in Petromyzontidael there is an arrangement that might be
said to be transitional between the teleostean and amphibian patterns.
In amphibians, all the vitelline cells, including those at the vegetal
pole, have a morphogenetic role but according to Weissenberg's recent
work (1934) in Petromyzon, the endoblast develops only from a submarginal
annular zone, while the cells of the inferior pole serve solely as a
food reserve, in the - same way as the unsegmented yolk of the teleost.
The differences that can be observed are purely quantitative. The
movements of convergence are much more marked in the trout, while ventral
1
Myxines, about whose gastrulation processes little is known, have
meroblastic eggs.
280
•
divergence is less distinct. A difference that at first appears more
striking is the time lapse between the formation of the primordia
anteriorly and posteriorly in the trout. Whereas in the amphibian the
neural plate appears very soon after gastrulation, in the trout it appears
even before the blastopore reaches the equator of the egg. While the
movements of gastrulation continue posteriorly, resulting in gradual
-elongation of the embryonic body, anteriorly all the organs, the neural
tube, notochord and somites, are developing and differentiating. This
difference is quantitative only. We know from the work of Vogt (1929)
and Bytel (1931) that convergence, invagination and extension are still
continuing at the edges of the punctiform blastopore when the neural
plate has appeared. The difference is, moreover, much more marked in
the large eggs of Salmonidae than in the smaller eggs of other teleosts
(see below). Although this special characteristic of embryogenesis in the
trout is not of supreme theoretical importance, it should be taken into
account. Luther's recent work (1935) shows that the very gradual progression
of gastrulation from anterior to posterior is paralleled by determining
events that take place just as gradually. The territories of the posterior
node and the neighbouring areas are still subject to regulation when the
anterior end of the embryo is already well differentiated. This parallelism
between morphogenesis and determination is in itself not surprising.
Van De Broek (1935) recently described the morphogenetic movements
of another meroblastic vertebrate, the selacian Scyllium canicula. He
observed the same components - invagination, convergence, extension,
epiboly, ventral divergence - always combined in the same way, which
seems to be typical of gastrulation in anamniotes. However, in Scyllium,
the ventral parts of the disk are purely epiblastic and undergo only a
combination of epiboly and divergence, without invagination.
281
•
Van De Broek also showed that at the time when the Scyllium embryo
begins to take shape, invagination has terminated at the dorsal lip
and, while gastrulation continues in the lateral areas, morphogenetic
movements in the area of the dorsal lip cease for a time. Not until
after this resting period do the axial territories begin to extend.
Van De Broek remarked that this observation was similar to those of
Wetzel on the primitive streak in the chicken. Without wishing to enter
into the degree of detail that will characterize the third paper in this
series, the author can state that his recent observations on bird eggs
leads him to a different conception of what is occurring. Although, as
Wetzel showed, the mesoblastic material of the primitive streak does go
through a resting stage (due, as it happens, to secondary causes that
will be analysed below), the subsequent drawing back of the primitive
node is merely the result of combined invagination and extension of
the notochordal material. The dissociation between invagination and
extension of the axial material seems to be a uniquely selacian
characteristic, and thus is no more common in birds than in amphibians
and teleosts. In the latter, as we have seen, invagination and extension
are combined until the end of gastrulation.
•
Van De Broek considers the dorsal lip of the blastopOre to be fixed
in Scyllium. Extension would therefore be posteroanterior, pushing the
head forward. This concept of growth in the forward direction in a
vertebrate embryo is difficult to accept. It is certainly not valid
for the teleost, in which both the persistence of the central vitelline
syncytium (Virchow) and the immobility of coloured markers placed at
the front of the head compel us to accept the cephalic extremity as a
fixed point. The invaginated material thus extends in an anteroposterior
direction, and the trunk also grows caudad.
;
•
282
The development of the teleost embryo is then, let us repeat, due
to extensive displacement of cell masses. Failure to take account of
this fact is liable to lead to grave errors..
Very recently, some American authors (Richards, Richards and Porter,
Richards and Schumacher, 1935) decided to study morphogenesis in two
specis of teleosts (Fundulus and Coregonus), by determining the mitotic
index (percentage of cells in mitosis divided by the total number of
cells) of the various territories at different stages. They based their
work on an exclusively American bibliography, with two exceptions, and
were seemingly unaware of the fundamental work of Vogt (1925, 1929). Their
.conception of the development of the body of the embryo can be summarized
in the following way. Morphogenesis implies growth. All growth is
accompanied by mitosis. Precise knowledge of the distribution of mitoses
in the various territories at the various stages of gastrulation should
thus provide a precise explanation of gastrulation. This author believes
these premises to be false, because mitotic activity does not necessarily
imply growth and the "growth" of the body of the embryo during gastrulation
is only apparent growthl due to cell movement. If they are indeed false,
the results obtained by Richards, Porter and Schumacher, no matter how
accurate, must have led them to absurd conclusions. They say, for instance:
"The mitotic_index of the germ_ring is low ... This might seem to indicate
that the germ ring does not supply any material for the eMbryonic shield
region. Il is constantly increasing in circumference and decreasing in
cross section and the lower mitotic rate here would not favour the transfer
of much needed material into the more active shield region. This evidence,
1
•
According to the measurements of Kopsch (1904), the volume of the embryo
of the trout, without the yolk, does not vary during the entire period
of gastrulation.
283
therefore, of the differentiation in the mitotic rate is not in l-Ine
the conception of the confluence of material from the
the
e embryonic shield region."
ring into
r an the y
y had merely examined the living
egg, as did their compatriot Mo If
arrived at more reasonable conclusions895)' these authors would have
their apparent
Moreover,
precision and their absolute
value, their results , despite
viewed with caution. (The results of this author, must themse
inlves be
have only relative value.)
published
1934 ,
Let us consider the data on which the above-quoted statement is based.
Here are the figures Richards and Porter obtained for Fundulus:
Cells
Mitoses
Counted
Index
Embryonic shield ectoderm'
.................
3577
125
3.49
•
809
27
3.00
.........
608
20
3.29
314
4
1.25
Primary endoderm2 of the embryonic shield .
Ectoderm of _ the ' enveloping border .
Endoderm of the enveloping border ..........
It is clear that although the total numbers of cells counted are large,
the numbers of cells in division are often so low that no real si nifi
can be attributed to the sometimes trivial differences in the mitotic
g
c.ance
indices.
The absurdity is even more apparent
conducted b
in the study of Core onus
by Richards and Schumacher. Their statistics were as follows:
3191 cells
19 mitoses
484
1001
5
329
I^
"
Although it is somewhat surprising that Richards and Schumacher claim
to have calculated an absolute value for the mitotic
one solitary instance of mitosis, it is not
•
Superficial layer
2
Chordamesoblast
index after countingg
all surprising that the
•
284
results for Coregonus and Fundulus do not agree.
If we consider only the well founded conclusions of these three
papers, there remains the following: the posterior node is not a focus
of cell proliferation. This finding in essence provides confirmation
of results published by the present author in 1934. Richards, Porter
and Schumacher find it to be in direct contradiction with the idea that
the posterior node is a "growth centre", as demonstrated experimentally
by Sumner (1904). As we have seen, knowledge of how the morphogenetic
movements occur makes it much easier to explain.
5. - Archenteron and roof of the archenteron
Endoblast and hypochord
•
•
One of the most curious features of development in teleosts is the
solid appearance of the developing organism, the absence of cavities.
Where in other forms we observe infolding followed by hollowing out, in
the teleosts we find compact buds of tissue, solid masses in which cavities
form secondarily by separation of cells.
There is, then, no archentric lumen in teleosts. This singular
gastrula is difficult to interpret. Ziegler (1902) thought of the archenteron
as being reduced to a fissure, with the roof being formed by the invaginated
layers and the floor by the yolk. This concept at first appears very
plausible, much more so than other theories (discogastrula of Haeckel, and
so on) and has the merit of allowing easy comparison between the amphibian
and teleost gastrulas. However, if it were correct, we would have to
accept that in the teleost the roof of the archenteron contains not only
.the part of the endoblast which constitutes the roof of the digestive cavity,
as in other vertebrates, but all the enteric material. We know from the
work of Henneguy (1888) that the endoblastic layer, which is at first
unicellular, converges upon the median line to form a rod in the middle of
•
285
which a lumen forms secondarily. To push to the extreme the similarity
between the teleost and the amphibian gastrula, it would be necessary
to suppose the virtual lumen of the archenteron to lie somewhere in the
endoblastic layer, instead of beneath it. Is the cavity of Kuprfer's vesicle
a vestige of the archenteric cavity, as many authors claim? Or is it
simply a cavity that plays some role in the nutrition of the embryo?
Its significance remains obscure. But is it truly useful to look for a
possible archenteric lumen in a solid formation such as the teleost
gastrula? The existence of an archenteron is absolutely necessary to
the enterocoelic theory. This theory, and the archenteron, will be
discussed below with reference to vertebrates as a group. In teleosts
in particular, no evidence in support of this theory has ever been found.
O
The morphogenetic movements take place in the same way in teleosts
as in amphibians. The chordamesoblast, which initially lies in the
marginal zone, reaches its ultimate position by moving around the lips
of the blastopore, and is therefore not of "gastral" origin, but of
"peristomial" origin. The concept of the gastral origin of the mesoblast
in amphibians has been upheld only because of faulty interpretation of
the appearance of the roof of the archenteron (see Vogt, 1929). In
Urodelae, this structure forms as follows. In the centre, the notochordal
primordium lies directly over the lumen of the archenteroh; laterally,
two endoblastic crests (Darmlippen) leave the vitelline . floor to
constitute the two lateral walls of the archenteron. They terminate beneath
the two lateral borders of the notochord. Between the endoblastic crests and
• the underlying mesoblast there is naturally a space. It is this space
that Hertwig (1906) erroneously took to be the lumen of the enterocoel
connected to the gastral cavity. Vogt (1929) quite rightly stressed that
•
286
the lumen does not separate a parietal layer from a visceral mesoblastic
layer but the entire mesoblast from the endoblast. This peculiar structure
of the roof of the archenteron, the significance of which is quite different
from that attributed to it by earlier authors, seems to be quite common
in vertebrates. It is found in cyclostomes, selacians, and reptiles. In
the latter, it arises as a result of a secondary process (see our
preliminary note, 1935). Is it also found in teleosts? Some authors,
such as Agassiz and Whitman (1884), Wilson (1889) and Sumner (1904) have
described it in the eggs of marine teleosts (Ctenolabrus, Schilbeodes).
According to them, the notochord in these forms is initially in contact
with the lumen of the archenteron, as it is with the roof of the archenteron
•in amphibians, and on either side of the notochord there are two endoblastic
lips which eventually fuse to form the roof of the digestive cavity.
Hoever, Boeke (1904) observed that in Muraenae, the endoblast consists,
from the beginning of development, of a continuous layer with no median
interruption. It is most curious that the American author Sumner, who
disputed Boeke's statements ideas on the basis of his own observations in
Schilbeodes, illustrated his article with drawings of Salvelinus
gastrulas, in which, from the earliest stages, the endoblast consists of
one continuous layer. The illustrations of his sections show that the
absence of a- median hiatus is not the result of an oversight on Sumner's
part. Sumner, however, has not a word to say about this contradiction.
The study of the early stages of the development of the endoblast
in teleosts is very laborious. It is a thin layer, often indistinct,
and there are not many embryos in which it can be seen to advantage.
In a careful study undertaken by this author, not one trout embryo was
found in which there was anything that could have been interpreted as a
median hiatus in the endoblast beneath the notochord. Figure 33a shows
•
287
an exceptionally clear transverse section of a very young embryo in
which the chordamesoblast has not yet separated into its constituent
elements. The endoblast forms a continuous layer, with no median
hiatus from one end of the embr o to the other. The author doubts
that there are originally two endoblas tic lips which join at an earlier
See photocopy of original
Figure 33 - (a) Transverse section through an embryo in which the
chordamesoblast has not yet separated from its constituent elements.
A continuous layer of endoblast can be seen; (h) Idem, near the posterior
estremity of the same embryo.
stage, because in cyclostomes, amphibians and reptiles, the lips always
join very gradually in an anteroposterior direction at a more advanced
stage than the one under consideration. If there were two endoblastic
•
J. PASTEELS
GASTRULATION DES VERTÉBRÉ JIIÉROBLASTIQUES
\loci' (1929) a insisté avec raison sur le fait que cette lumière
ne sépare pas un feuillet pariétal d'un feuillet viscéral
mésoblastique mais bien tout le mésoblaste de l'entoblaste.
Cette structure particulière de la voûte archentérique, à
laquelle on peut donc attribuer une signiEcation tout autre
que celle des anciens auteurs semble assez générale chez les
Vertébrés..On la retrouve chez les Cyclostomes, les Sélaciens,
les Reptiles. Chez ceux-ci, elle est acquise à la suite d'un
processus secondaire, (cf. notre note préliminaire, 1935).
La retrouve-t-on également chez les Téléostéens ? Certains
aiTtedrs, tels que AGASSIZ et WHITMAN (1884), H. V. WrLsox
(1889), SUMNER (1904), la décrivent chez des ceufs de
Téléostéens marins (Ctenolabrus, Schilbeodes). La chorde y
serait, comme à la voûte' archentérique des Amphibiens,
primitivement en contact avec la lumière archentérique,
et de part et d'autre de la chorde existeraient deux lèvres
-entoblastiques qui par la suite se fusionneraient pour former
le plafond de la cavité digestive. Cependant, chez les
Murénohles, BOEKE (1904) observe que l'entoblaste est
constitué dès le début du développement par une couche
continue sans aucune interruption médiane. Le plus curieux
est que SUMNER, qui s'élève contre cette assertion de BOEKE,
en se basant sur ses observations chez Schilbeodes, figure
dans le mèrrie travail des reconstructions graphiques de
gastrulas de Salvelinus où, depuis les premiers stades, Pentublaste est représenté . par une lame continue ; ses „-,,ures de
coupes m
. ontrent d'ailleurs que l'absence d'hiatus médian
n'est nullement le fruit d'une inadvertance de cet auteur
américain ; dans le texte de ce travail, on ne trouve mot sur
cette contradiétion.
L'étude des premiers stades de la formation de l'entobluste
clicz les Téléostéens est en réalité très pénible,. Ce feuillet
minee est souvent . fort inLlistinet et rares sont les embryons
ciui un donnent une vue favorable. Une étude t' in attenriv2
ne nous a montré Cil anC1111 Ca., CliCZ la Truite, des aspel,
pou\-,Int s'interpréter oriltne un hiatus médian de l'en tublaste sous la chorde. En revanche, le cas exceptionnellement
clair de la figure 33a montre une coupe transversale d'embryon très jeune chez lequel le chordo-rnsoblaste ne s'est
pas encore séparé en ses éléments constitutifs. L'entoblaste
y forme une lame continue, sans hiatus médian, d'un bout ci
l'autre de l'embryon. La suture des lèvres entobfastiques
éventuelles s'est-elle déjà faite à un stade précoce ? Nous
,
.
4
-,:z•
-• •
(".%
•
••• •
987
'•
7.:”;
Th
c...
•
FIG. 33. — •2) Coupe transversal:3
travers un rnLrvc. jont 1:t."
:IO Aï: dié en ses é!énints. 1! existc
• une. iarne continue d'ente.blaste. b)
prz:.s
., -......17_;riettre du 1ni embryon.
•
chordo - inésr.iblaste n'est pas
en doutons pour la raison suivante. Cette suture se fait
toujours (Cy,:lostornes, .-‘mphibiens, R2ptiles) très prigressivement d'avant en arrière à un stade plus avam:é
que cerui
que . nous figurons. Si réellement des lèvres entoblastiques
•
288
lips in the trout, in which the anteroposterior progression of
morphogenesis is particularly slow, they would have to be apparent
in the posterior part of the above embryo. The section reproduced
in Figure 33b shows no such thing. Should we then conclude that
the observations of Wilson and Sumner were inaccurate?
•
•
Those of Wilson's illustrations that were reproduced in
Hertwig's Traité (1906, V 1, p 815), seem particularly convincing
but are too diagrammatic, as any observer of teleostean development
will agree. Sumner's work, as has been mentioned several times, is
of exceptional quality. However, it seems difficult to accept that
the endoblast can form differently in different teleosts. The question
therefore remains open. Let us remark, though, that it would not be
surprising to find a different arrangement in the deep layers of the
teleost than in the roof of the archenteron of other vertebrates. It
was stressed above that the two deep layers resting on the yolk, the
chordamesoblast and the endoblast, are not directly comparable to the
roof of an archenteron because they contain all the endoblastic
material, including that of the digestive cavity.
As Vogt showed,"the fusion of the endoblastic crests- beneath
the amphibian notochord is only one stage in the processes of convergence
which continue until the end of gastrulation in the roof of the
archenteron. The same type of convergence occurs in the deep layers
of teleost embryos. The condensation of the endoblastic layer into a
median rod and the condensation of the initially spread out mesoblast
into paraxial somitic masses are familiar processes (Henneguy, 1888;
Swaen and Brachet, 1899, 1901) which it is not necessary to discuss
at length. But the real significance of these transformations should
•
289
be stressed. They result from late convergence of the axial organs,
a process directly continuous with the movements of convergence that
we studied in the earlier stages.
The author would also like to make an important rectification to
some of the observations made by earlier workers about the hypochord.
This organ, transiently present beneath the notochord, was almost
unanimously considered to arise from the endoblast (St-iihr, 1895;
amphibians; Henneguy 1884, Wilson 1899, Franz 1897: teleosts; Salensky,
1882: ganoids). Only Perenyi (1887) considered the selacian hypocord
to be of mesoblastic origin. Discordant though this idea was, in
1929 Vogt extended it to amphibians. Vogt was able to show clearly
that the hypochordal rod arises through convergence of the inside edges
of the somites, which join beneath the notochord. In Anurae (Bombinator),
the situation is somewhat different, since the hypochord consists from
the beginning of development of a single rod filling the endoblastic
hiatus. Using coloured markers, Vogt and his student Mayer (1931) traced
the origin of the hypochord, whose anlage is close to that of the notochord,
and showed that its apparent relationship with the endoblast is only
secondary. More recently still, Hayek (1931) returned to the descriptive
study of the formation of the hypochord in all chordates._ His results are
of great value; they show that the observations of Vogt can be extended
to all anàmniotes. In Amphioxus, as in selacians, dipneusts and
amphibians there are "parachordal cells" that separate from the initial
chordamesoblastic mass and subsequently join underneath the notochord,
where they temporarily fill the endoblastic hiatus between the
"Darmlippen". When the endoblastic lips meet, they push the parachordal
elements upward, where they come to constitute the hypochord. The last
•
290
stage of this complex process is always the clearest, which explains
why earlier authors took the hypochord to arise from the endoblast.
As Hayek was able to show, in the anurean Bombinator, the hypochordal
layer, which initially seems to be a single entity, probably arises
from the very early fusion of paired parachordal elements in the still
unsegmented chordamesoblastic layer. Within the anurean order, there
is a clear transition in Rana between the Bombinator type of development and the urodelan type, which is found in the anurean Bufo. In
his study of teleosts, Hayek, for lack of material, had to limit himself to interpreting the drawings of other authors (Henneguy,
Goronowitsch). On this basis, Hayek supposed there to exist in the
trout, as in Bombinator, a hypochordal layer separating the notochord
from the median endoblastic hiatus.
This is not the case at all, as the following observations show.
At the left of the section shown in Figure 34 one sees, just as in
•
See photocopy of original
Figure 34 - Transverse section showing the formation of the hypochord.
•
(.•
20
J. PASTEELS
•
DES VERTÉBRÉS itIÉROBLASTIQUF.S 291 -.. '
gauche, on voit, toUt comme chez les Urodèles, une languette
venant de l'extrémité interne du sornite. La série des coupes
prouve bien que ces languettes, provenant de part et d'autres
des somites, se détachent et se rejoignent sous la chorde.
Dans la suite du développement, la lame hypochordale
s'arrondit pour former la tige caractéristique. Malgré cette
rupture, comme chez les Urodèles, persistent des filaments
qui relient les deux organes. Cette évolution se poursuit
d'avant en arrière. Encore une fois,' cette progression antéropostérieure est beaucoup plus étendu e chez la Truite que
chez les Amphibiens. A un stade beaucoup plus avancé que
celui des figures précédentes, alors qu'a, l'extrémité antérieure du tronc l'hypochorde est déjà bien constituée, nous
voyons un peu en avant du nœud postérieur, sur la fig. 32d,
une sorte de sangle embrassant le contour inférieur de la
chorde. Un peu plus en arrière encore, en 32c, nous trouvons
cette première ébauche hypochordate en continuité directe
avec les somites.
GASTRULATION
cordon hypochordal. Cette dernière étape de cette évolution
complexe est toujours la plus nette, ce qui explique aisément
l'illusion des anciens auteurs qui voyaient dans Phypochorde
une formation entoblastique. Comme HAYEK a pu le montrer,
la lame hypochordale qui semble primitivement impaire
chez l'Anoure Bombinator doit sans doute résulter d'une
fusion très précoce, dans la lame chorclo-mésoblastique
'encore insegmentée, des éléments pairs parachordaux. En
effet, à l'intérieur du groupe des Anoures, op trouve chez
Rana une transition très nette entre le type Bonzbinator et
celui des Urodèles, retrouvé chez l'Anoure Be. En ce qui
concerne les Téléostéens, HAYEK, faute de matériel, a dû
se borner à interpréter les images des auteurs (HENNEGUY,
GORONOWITSCH). D'après celles-ci, HAYEK. suppose qu'il
existe chez la Truite, une lame hypochordale séparant la
chorde de la fente médiane entoblastique comme chez
Bornbinator.
Il n'en est rien, comme le montrent les observations suivantes. Sur la coupe représentée sur la figure 34, à
Bref, la formation de l'hypochorde chez la Truite présente .
unealogifrptvcequdériHAYEKchezl
• d
e,
;•, 0 ...ef)g.
•-;
-
.."`•
.
Z;:)-(
' z') ‘%•- •
'-"
• --,
!
"
•
'57
,‘
C;'-D•;(,•••
•---.1,12;;-'f;', 5"., Y%-e§, • - -I,
• ,
!:;e:
(•••
‘`), !•à
Cyclostome. En avant du tronc, comme chez les Urodèles,
deux languettes se séparent du" mésoblaste et viennent se
soucier sous la chorde ; en arrière du tronc cette soùdure est •
réalisée déjà avant la rupture entre sornites et éléments
parachordaux.
Cependant,
existe une profonde différence entre les
rapports des éléments paraehordaux de la Truite et ceux
des autres Anamniotes. Chez notre Téléostéen, Phypochorcle .
estoujrxcldavûtehnériquplame
entoblastique continue. La participation temporaire de ces
éléments parachordaux à la paroi dorsale de la cavité digestive,
telle qu'elle est réalisée chez les autres Anamniotes, ne peut
donc étre considérée comme un caractère général.
•
-
•
FIG. 34. — Coupe 'transversale montrant la formation de l'hypo-
chorde.
291
Urodel.ae, a little tongue of tissue coming from the internal edge
of the somite. The series of sections proves that these tongues,
coming from the somites on either side, become detached and join
beneath the notochord.
The hypochordal layer subsequently rolls up
into the characteristic rod.
Despite this rupture, filaments joining
the two organs persist, as in Urodelae. Development of the hypochord
proceeds in an anterior to posterior direction. Once again, the anteroposterior progression is much slower in the trout than in amphibians.
In the more advanced stage shown in Figure 32d, when the hypochord is
already well formed at the anterior extremity of the trunk, a type of
band can be seen around the lower edge of the notochord, a little
anterior to the posterior node. Further posteriorly still, in Figure
32c, the early hypochordal primordium can be seen, directly continuous
with the somites.
•
In short, the development of the hypochord in the trout is
strikingly similar to that described by Hayek in cyclostomes. As
occurs in Urodelae, two tongues of tissue separate from the mesoblast
in the anterior part of the trunk and fuse beneath the notochord. In
the posterior part of the trunk they join before the somites and
parachordalelements_separate._.
Nevertheless, the relationships between the parachordal elements
in the trout are profoundly different from those in other anamniotes.
In the trout, the hypochord is always separated from the roof of the
archenteron by a continuous layer of endoblast. The temporary inclusion
of the arachordal elements in the dorsal wall of the digestive cavity
that is seen in other anamniotes cannot, therefore, be considered a
universal feature.
0
•
292
III - CONCLUSIONS
1- Gastrulation in the Trout
As explained above, the point of this exercise in prospecting
the territories of the blastula by means of vital stains was to
determine the distribution of the prospective organs on the surface
of the egg and to determine the movements by which the anlagen are
brought into place in the embryo. Now that the results have been
described and analysed, the author proposes to review them briefly,
examining the successively changing locations of the primordia at
the principal stages of gastrulation.
•
Figure 35A shows the organ-forming regions on the surface of
the blastula. The following figures illustrate the successive
alterations in the topography of the blastula and the progression
of the morphogenetic movements which cause the displacement. In
these drawings, the embryos are outlined in heavy lines, the organforming territories in light lines; the notochord is indicated by
vertical bars, the nervous system by horizontal bars, the cephalic
territories of the prochordal plate and the brain by denser bars
than the notochordal and neural territories of the trunk, the somites
by diagonal bars, and the lateral and ventral mesoblast by dots. The
movements of the superficial layer are indicated by full arrows and
those of the already invaginated chordamesoblast by broken arrows.
Figure 35B shows the beginning of invagination. The prochordal
plate has sunk inward, as have the anterior extremity of the notochord and the first myotomes. These territories near the posterior
median extremity of the blastodisk are beginning to converge.
Convergence and invagination have extended to the entire periphery of
the disk in 35C. What is left of the notochord on the surface is
moving closer and closer to the median line. The characteristic
/
1
•
293
See photocopy of original
Figure 35 - Diagrams showing the movements of the organ-forming
territories during gastrulation. Heavy lines: outline of the embryo.
Light lines: outlines of the territories (vertical bars: notochord;
dense vertical bars: prochordal plate; horizontal bars: nervous system;
dense horizontal bars: brain; diagonal lines: somites; dots: lateral
and ventral mesoblast). Full arrows: movements in the superficial
layer; broken arrows: movements in the deep layers.
movements of extension Of the median territories and convergence of
the lateral parts are apparent in the nervous system, mainly in the
truncal nervous system. The lateral marginal zone undergoes the
J. PASTEELS
-
•
GASTRULATION DES VERTÉBRÉS MÉROBLASTIQUES
293
CONCLUSION S
I. — Syntlti.e,e de la Gastrulation de la Truite. .
Notre étude de prospection des divers territoires du germe
au moyen de marques colorées devait, comme nous le disions
plus haut, nous mener à deux ordres de résultats a) la
;
répartition des organes présomptifs à la surface de
la
mise
en
évidence
des
mouvements
qui
mettent
ces
b)
ébauclies en place dans le corps embryonnaire. Ces résultats
étant connus et analysés, nous nous proposons dans les
lignes suivantes d'en faire une brève revue en examinant
les changements successifs des localisations des ébauches
aux stades les plus caractéristiques de la gastrulation.
La figure 35A nous représente les territoires présomptifs
-à la surface de la blastula. Sur les figures suivantes, nous
tâcherons de suivre *les remaniements successifs de la
topographie des ébauches et le déroulement des mouvements
rnorphogénétiques exprimant leurs déplacements. Sur ces
figures, les formes des embryons sont en traits accentués, les
contours des territoires en ttaits minces la chorde est
hachurée verticalement, le système nerveux horizontalement,
les territoires céphaliques de la • plaque préchordale et du
cerveau sont en hachures plus intenses que les territoires
•chorclaux et neuraux du tronc ; pour le mésoblaste •: hachures
Obliques pour les somites, ponctuation pour les lames
latérales et le mésoblaste ventral. Les mouvements sont
indiqués en grosses fleches ; celles-ci sont pleines pour le
feuillet superficiel, en tirets pour le chordo-mésoblaste
déjà invaginé.
•
La figure 35,B montre un début d'invagination. La plaque
préchordale a disparu dans la profondeur, ainsi que Pextré, mite antérieure de la _chorde et les premiers myotomes ;
ces territoires voisins de l'extrémité postérieure et médiane
du blastocliSque commencent' à converger. Convergence et
invagination se sont étendus à toute la périphérie du disque
en 35,C. Ce qui reste de la chorde en superficie se ramasse
-
FIG. 35. — Schémas montrant les déplacements des territoires au
cours de la gastrulati.m. Contours épais : formes de l'embryon. Contours minces : limites des territoires (hanures vertkales : chorde ;
hachures verticales danses : plaque prézhordale hachures horizoiitales : systè!1e. nerveux; hachures horizontales denses : cerveau ;
li"..,nes obliques : somites; pointillé : iné-soblaste latéral et ve.ntral;
Fléches pleines : mouvements :Jans le feuillet superficiel. Flés:h-zs
interrompues : mouvements dans I feuillets prof.):1,1,.
de plus en plus vers la ligne médiane. Les mouvements
caractéristiques d'extension des territoires médians et de
convergence des parties latérales se fout sentir dans le
Système nerveux, surtout troncal. Quant à la zone marginale
294
•
•
combination of convergence and invagination that was analysed above;
the movement of convergence is apparent both before and after the
area rolls up around the lip of the blastopore. Note that at this stage
convergence is most marked in the lateral parts of the enveloping
border. In the following stage (Figure 36D), the entire enveloping
border, both ventrally and on the sides, becomes very stretched out.
At the same time as the movements of convergence are bringing the
territories, both superficial and deep, toward the body of the embryo,
epiboly is causing progressive dilatation of the enveloping border that
encircles the vitelline mass. In the body of the embryo, which is
beginning to elongate, these movements are becoming more intense, and
the further component of extension has entered the picture. Extension
can be seen much more clearly in 36E. In both the superficial and the
deep layers, the median territories have extended markedly toward the
caudal end. At the posterior extremity, at the le\iel of the terminal
node, the most posterior part of the notochordal primordium is still
invaginating. In the surface layer, the movements of convergence are
still bringing the lateral and posterior parts of the nervous system
toward the median line (right side of figure) and in the deep layer,
these movements are having the same effect on the invaginated mesoblast,
which will eventually lie on either side of the notochord - (left side
of figure). - Thé same processes are continuing in 36F. This stage is,
however, characterized by a few new features. The organs of the anterior
part of the trunk are in place and the entire notochord has invaginated.
Laterally, some mesoblast remains in the superficial layer of the
enveloping border. While this mesoblast invaginates slowly, the mesoblast
which has already reached the deep layer of the enveloping border comes,
through continuing convergence, to lie on either side of the notochord,
which is gradually elongating owing to the extension of the material
of the posterior node. Concurrently, in the superficial layer, the
posterior and median neural material is extending caudad while the two
lateral horns come round to the sides.
295
See photocopy of original
....
Figure 36 - Continued from Figure 35. In E and F the movements of the
neural territory are indicated on the right only and those of the mesoblast on the left only.
•
•
294
J. PASTEELS
•
latérale, elle subit le complexe de convergence et d'invagination que nous avons, analysé plus haut ; le mouvement de
cônvergence se fait sentir autant avant qu'après l'enroulement autour de là lèvre blastoporale. Notons qu'à ce
stade la convergence est surtout .intense dans les parties
latérales du bord d'enveloppement. Au stade suivant
• (fig. 36,D), c'est tout le bord d'enveloppement aussi bien
ventralement que sur les côtés qui est soumis à un étirement
intense ; tandis Que les mouvements de cdnvergence
ramènent continuellement les territoires, tant superficiels
que profonds, vers le corps embryonnaire, l'épibolie dilate
de plus en plus la ceinture du bord d'enveloppement autour
de la masse vitelline. Au niveau du corps embryonnaire
qui commence à s'allonger, les mouvements précédemment
décrits s'intensifient ; de plus une nouvelle composante
entre en scène : l'extension. Celle-ci sera bien plus nette en
•
36,E. Tant dans le feuillet superficiel que dans le'kuillet
profond, les territoires médians s'étendent fortement vers
l'arrière. 'l'Out en arrière, au niveau du noeud terminal, là
partie toute postérieure de l'ébauche chordale s'invagine
encore. -Les mouvements de convergence entraînent toujours
vers la ligne médiane les parties latérales et postérieures du
système nerveux en surface (côté droit de la figure), et en
profondeur, le mésoblaste invaginé qui va se placer sur les
côtés de la chorde (côté gauche de la figure). Les mêmes
processus continuent en 36,F. Ce stadese caractérise cependant par quelques nouveautés. Les organes de la partie
antérieure du none sont en place et toute la chorde est
invaginée. Latéralement cependant, il reste encore du mésoblaste dans le feuillet superficiel du bord d'enveloppement.
Tandis que ce mésoblaste s'invagine lentement, celui qui
est déjà situé dans le feuillet profond du bord d'enveloppement, par une convèrgence continuelle, vient se placer de
part et d'autre de la chorde qui se prolonge a u fur et à mesure
par l'extension du matériel du nœud postérieur. Parallèlement, en superficie, le matériel neural médian et postérieur
s'étend vers l'arrière tandis que les deux cornes latérales
FIG. 36. — Suite de 1:t fig. 35. Mêmes indications. En E et F /es
mouvements du territoire neural ne sont indiqués qu'A droite. .7eux
, du mésoblaste A p.auche seulement.
296
These movements continue until all the organs have been brought into
position, a stage which probably coincides with the closure of the
blastopore, although its exact time of occurrence could not be pinpointed. While anteriorly the embryo is already formed and beginning
to differentiate, posteriorly the primordia continue to be laid down
according to the same principle.
2 - Extension to other teleosts
Are these observations applicable to teleosts in general or only
to the trout egg? On first consideration, the interspecies differences
in gastrulation are much more marked in the teleosts than in any other
order of vertebrates. But brief examination shows that these so
obvious differences are due solely to the size of the yolk that the egg
must incorporate. The trout egg is a large egg. This is the reason
that morphogenesis in the trout is very gradual and that the movements
of gastrulation are still proceeding step by step in the 13osterior part
of the embryo when the embryo and all its organs are fully formed
anteriorly. This type of development is not characteristic of teleosts;
it is only rarely seen among them. It is characteristic of Salmonidae
and Siluridae. In some other forms, the yolk is larger still and
morphogenesis comes to an end before the blastopore closes. This is the
case in Batrachus tau (Clapp, 1891; Sumner, 1894) and in Gymnarchus
niloticus (Assheton, 1907) 1 .
According to these authors, in the above two species, but particularly
in Gymnarchus, when the embryo is already fully formed, the blastopore
is still wide open and it surrounds the inferior part of the yolk. The
round blastopore becomes oval, then linear, and closes by a process that
is definitely concrescence.
1
Quoted from Ziegler (1902)
•
297
This extraembryonic concrescence is only a secondary, adaptive process.
In most species, however, the yolk is much smaller with respect to the
embryonic primordium. The sketches of living gastrulas in Figure 37
are taken from Chevey's study of the perch (1925). Figure 37a depicts
early gastrulation. The embryonic part of this perch gastrula is
entirely comparable to stage C of our Figure 1, but half the yolk has
already been incorporated into the embryo. The embryo of (d), which
has almost completely incorporated the yolk, is at the same stage as
the trout embryos shown in figures 2F or 2G. It is clear that although
the morphogenetic movements of the various teleosts must be the same
in principle - and the similarity between the purely embryogenic parts
See photocopy of original
Figure 37 - Sketch of gastrulation in the perch, taken from Chevey, (1925).
296
9
.
.
J.
PASTEELS
.
•
.
GASTRULATION DES VERTÉBRÉS
enflent se rappliquer sur ses côtés. Les mêmes mouvements
vont se continuer jusqu'à la mise . en place de tous les organes
— ce qui doit vraisemblablement coïncider (nous n'avons pu
préciser exactement ce point) avec la fermeture du
blastopore. Tandis que l'embryon déjà constitué se différencie
de plus en plus en avant, en arrière la mise en place des
ébauches se continue toujours selon le même principe.
MROBLASTIQUES 297
aucun doute. Faut-il ajouter que cette concrescence extra
embryonnaire n'est qu'un processus secondaire d'adaptation?
Chez la plupart de3 espèces cependant, le vitellus est
beaucoup rnoins abondant par rapport à l'ébauche embryonnaire. Nous empruntons la Figure 37 au mémoire de
CHEVEY (1925) sur la Perche. Ces croquis de gastrulas faits
d'après le vivant nérus montrent en a un stade de début de
2. — Éxtenft:ion aux autres Téléostéens
•
•
. -
••
•
. Ces- observations sont-elles applicables aux Téléostéens;
en 7,érieral ou doivent-elles être réservées au seul, ceuf de
Truite ? A première vue, les différences entre les gastrulations
des diverses espèces sont beaucoup plus accentuées chez les
Téléostéens que chez n'importe quel autre ordre de Vertébrés
Mais un examen rapide suffit à démontrer que ces différences
s) apparentes trouvent leur seule raison d'être dans la taille
de la masse vitelline que l'oeuf doit englober. L'oeuf de Truite
est un très gros oeuf. C'est pour cette raison que la morphogenèse est très progressive et que, pendant qu'en arrière
de l'embryon la dynamique de la gastreation se poursuit
pas à pas, en avant, l'embryon est déjà constitué avec tou..;
ses organes. Une telle évolution, loin d'être la caractéristique
des Téléostéens, n'y est qu'exceptionnelle. Elle caractérise
les Salmonidés, les Siluridés. Chez quelques autres formes,
le vitellus est encore plus développé et la morphogénése
prend fin avant la fermeture du blastopore. 11 en est
ainsi chez Bafrachus tau (CLAPP 1891, SUMNER 1904), chez
Gyinnarchus nilbticus (.31SSHETON 1907)
D'aprèS les descriptions de ces auteurs, les œufs de ces
deux espèces, surtout Gymnarchus, au moment où l'embryon
est entièrement constitué, laissent encore apercevoir un
blastopore béant . , entourant la partie inférieure du vitellus .
Lafermtudcblsop,eivmntarod
ovalaire, linéaire, se fair par une concrescence qui ne fait
(') Cité d'après ZIEGLER.
(1902).
FIG.
37. — CrcquiS
gastrula:ion
1.I Perche.
C2F.:
( 1 92 5).
gastrulation, entièrement comparable, en ce qui concerne
la partie embryonnaire, au stade C de notre figure 1. Or,
déjà chez la Perche, .le vitellus est à demi englobé par l
bryon. L'embr:y'on d, où le vitellus est déjà presque complétement enfoui, correspond aux stades F ou G (rig. 2) de la
Truite. 11 est clair que si les mouvements morphogénéti..;ucs
doivent: en principe étre les Inémes -- et la similitude
des: aspects des parties purement embryogènes des divers
298
of the different species, disregarding their relationships with the
yolk, leaves no doubt of this - they must take place in more compact
fashion in forms such as the perch. There is no need for interminable
convergence to bring the material spread out thinly around an enormous
yolk back in toward the embryo. On the other hand, it is probably that
the mesoblastic material has not finished invaginating when incorporation
of the yolk is complete, and that coloured stains applied around the
punctiform blastopore would reveal late invagination at this time.
Thus, the differences between the morphogenetic movements of the trout
and those of amphibians in terms of time relationships would probably
not be noticeable in teleosts with small eggs, so that the similarity
between morphogenesis in these species and in amphibians would be striking.
3 - General
The author will not be able to put forward a general theory of
gastrulation in chordates until the end of this series of studies on
mesoblastic eggs. However, what is already known of gastrulation in
amniotes can serve as the basis for a preliminary discussion, the
conclusions - of Which can be reviewed later.
The differences between the amphibian gastrula and the teleostean
gastrula are striking. The former has an archenteron, a huge cavity
which communicates with the exterior through the blastopore; the latter
has no archenteron (as we have seen, the potential space between the
blastodisk and the yolk does not truly fit the definition of archenteron)
and the blastopore, plugged by the yolk, is also closed off as a result
of the adhesion of the covering layer and the vitelline mass. Yet there
•
299
are strong similarities between the two types of gastrulation; the
arrangement of the anlagen and the movements through which the anlagen
are brought into place are almost identical in both types of anamniotes.
In light of the above, one cannot help regarding the classical
theory of the gastrula, or rather of the gastraea, as slightly suspect.
Haeckel, Lankester, and others, established the idea that all metazoa are
characterized from the beginning of development by a gastrula stage,
a didermic stage itself characterized by an archenteron with the
blastopore opening into it. Haeckel felt this stage to represent a
hypothetical ancestral form, the gastraea. There is no doubt that this
idea made it possible to conceive of the primordial morphogenesis of
all metazoa as following the same general pattern and that for this
reason it was remarkably useful. But other authors, such as Brachet
(1921), have pointed out the difficulties one encounters when one trieS
to liken the gastrulean forms of all vertebrates to the hypothetically
archaic form found in Amphioxus. Without dwelling on the phylogenetic
aspect of the question, Brachet stated that "the concept of the gastrula
remains very fruitful if one views takes a wider view of it than
Haeckel did and. leaves it vaguer, if one can grade its characteristics
according tO their real order of importance." Brachet then gave the
following definition: "Gastrulation is the process whereby there forms
from the mass of cells produced by cleavage of the egg an embryo with
two layers, one internal and one external, which remain continuous with
each other at a given point along their extent."
This concept of the didermic gastrula, which was shared by Hubrecht
and Keibel, was severely criticized by Vogt (1929). It is incorrect,
as we know today, in that it implies that the notochord and the mesoblast can be derived from either layer and of course, in that it excludes
the formation of the chordamesoblast from gastrulation. A gastrula does
300
not consist of ectoblast and endoblast, but of ectoblast, endoblast and
chordamesoblast. The three germ layers are present and autonomous from
the beginning of gastrulation, as will be further discussed below. To
reconcile these new data with the older theories, Vogt (1929) conceived
of a gastrula-gastraea composed of an external and an internal layer
and-a marginal, blastoporal territory as well. Vogt thus continued to
subscribe to the Haeckel's theory that the archenteron was the primitive
digestive cavity.
It is clear that the essential features of gastrulation in vertebrates,
to confine ourselves to these for the moment, have to be found in all
animals. But the archenteron is a singularly inconstant feature. It
is well developed in prochordates and in most anamniotes but not apparent
in teleosts. Of the amniotes, birds and mammals have no archenteron at
all; the others display only the small notochordal canal, the vestige of
the archenteric canal of reptiles. In 1935, the author showed that the
latter is purely chordamesoblastic and comes into contact with the endoblastic walls only secondarily, through the formation of a fissure in its
floor. The concept of the archenteron as the primary digestive cavity is
essential only if one believes in the enterocoelic theory, in the
gastral" origin of the mesoblast. But we know from the work of Vogt
(1929) thàt Hertwig's belief (1906) that in amphibians, the coelomic
pouches separate from a single original archenteron was incorrect, and due
to misinterpretation of some aspects of transverse sections. The fissure
that supposedly represented the enterocoelic pouch did not separate,
as has had been thought, a visceral mesoblastic layer from a parietal
mesoblastic layer but the entire mesoblast from the endoblast. Hertwig
(1901) and his student Gehrardt (1901) described coelomic pouches on
either side of the notochordal anlage in reptiles, which they observed
301
on transverse sections. This author showed (1935) that these supposed
pouches are only transient folds arising from the movement of the mesoblast after the fissure forms in the floor of the archenteric canal.
In amphibians and reptiles, as in teleosts and even prochordates, the
chordamesoblastic material is brought into position by invagination
around the lips of the blastopore and is thus of "peristomial" origin.
The evidence is even more convincing when the processes by which the
endoblast and the chordamesoblast brought into position are dissociated
from the beginning of gastrulation, as in Urodelae and amniotes
(Vogt, 1929).
In prochordates and cyclostomes, there is definitely a single original
gastrulean cavity from which the chordamesoblastic and enteric primordia
seem to separate secondarily. Weissenberg (1933), basing himself on
observation of gastrulation in Petromyzon, stressed the fact that in
the latter species as in Amphioxus, the mesoblast is an "echter
Urdarmabkommling", a true archenteric derivative, and that consequently
the "gastral" origin of the chordamesoblast represents an earlier pattern
of development than the peristomial origin demonstrated in amphibians
by Vogt.
Van De_Broek (1935), withOut being as explicit, pointed out the
close relationship that seems to exist between the endoblast and the
chordamesoblast of Scyllium. He felt that "the chordamesoblast appears
to have more affinity with the endoblast than with the ectoblast". 1
Let us point out, once again, that it is of primary importance to
remain within a general frame of reference and that any definition of
the development of the chordamesoblast must be applicable to all chordates.
If we accept that the chordamesoblast arieses from the endoblast, how
1
•
Translated from the Flemish
302
can we explain that in Urodelae (Vogt, 1929), the two layers separate
from each other and follow different paths from the very beginning of
gastrulation? How can we accept that in Urodelae the mesoblast is an
archenteric derivative when it was never part of the archenteron? And
how can we accept that the very clear arrangement of the layers in
Urodelae is the result of a secondary modification? If the connections
between the mesoblast and the walls of the archenteron can disappear
so easily, it is because they are of no theoretical importance.
The archenteric cavity that characterizes Amphioxus seems, it is
true, to form from enterocoelic pouches. But We now know from the work
of Conklin (1932), of which Weissenberg was unaware, that the prospective
mesoblast of Amphioxus, like that of ascidians, forms a ventral crescent
on the surface of the blastula which, together with the dorsal notochordal
crescent, constitutes a peristomial ring as in amphibians. We also now
know that gastrulation in Amphioxus and ascidians differs only in slight
details, such as the size and the number of cellsl from gastrulation in ascidians; that in both forms organ-forming territories whose fate is
already determined, at least in part (see the experimental work of
Conklin, Dalcq, Tung), exist on the surface of the blastula even before
gastrulation begins, and that the wall of the gastrulean -cavity, far
from representing a morphological cavity, is a disparate'entity.
Thus, as Rabl predicted in 1916, chordamesoblastic, endoblastic,
and ectoblastic territories can be distinguished on the surface of both
the prochordate and the vertebrate blastula. The periods during which
the first two layers sink into the egg may overlap for a time as in
cyclostomes, selacians and teleosts, but in other cases, the two layers
may differentiate from the outset, as in Urodelae. In oligolecithal
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303
chordates, the cell movements of gastrulation may produce an infolding
from which arise à the archenteric cavity, a transient mosaic of independent
anlagen. In mesoblastic vertebrates (teleosts, primitive streak amniotes),
the layers slide over one another without a cavity becoming interposed.
•
The archenteron, an inconstant' and disparate cavity, is a weak basis
on which to define gastrulation. The blastopore, however, is uncontestably
a constant entity; it is found, without exception, in all chordates and
invertebrates. Care must be taken not to define the blastopore according
to its etymological sense (pore = mouth). Wetzel (1929) based himself on
such a definition 2 when he took a stand against the traditional idea that
the primitive streak was like a true blastopore, which had been taken up
by Gr'Aper, the Raubers, Balfour and Van Beneden. When there is no
archenteron the blastopore cannot be thought of as a mouth, but it
always serves as the orifice through which the chordamesoblast in chordates
and the endoblast in prochordates and anamniotes are brought inside the
blastula. The reason it is difficult to define the gastrula is that
gastrulation is a dynamic process. The definition of gastrulation must
be expressed in dynamic terms: the migration and bringing into position
in the body of the embryo of the various territories originally distributed
on the surface of the blastula. The movements by which the territories
are brought into position are the morphogenetic movements -which, with some
slight variàtions, are the same in all chordates: invagination, epiboly,
dorsal convergence, extension, and to a lesser extent, ventral divergence.
They alone characterize gastrulation.
The notion of concrescence arose through fallacious interpretation
of convergence; no such process exists. That it does not occur universally
•
1
The inconstancy is even more striking in invertebrates; one wonders
what could be represented as the archenteron of a polychaete or an insect.
2
"Der Streifen ist kein Urmund" (Wetzel, 1929). (The streak is not a
primitive mouth.)
•
304
has been apparent since the work of Vogt (1929: amphibians). Now
that it has been found not to occur in teleosts, the very group in
which His thought he observed it (1873), the concrescence theory has
been delivered the coup de grâce and should be forgotten forever.
The same applies to the many variations on the notion of "growth
centre". To begin with, nothing is less precise than the term "growth"
when one is speaking of embryonic development. Although most authors
take the word to mean an increase in mass through cell proliferation,
others use the word differently. Kopsch (1904) does not explicitly
state how he believes the growth centres to function, but since his
measurements showed that the volume of the embryo does not vary
appreciably during gastrulation, he can hardly feel that cell proliferation
is occurring.
Whatever the case, all the authors who believed in growth centres
(Kopsch, Hubrecht, Assheton and more recently Veit) always had in mind
selective localized centres for the different parts of the body
(Hubrecht's theory of cephalogenesis and notogenesis, Assheton's theory
of protogenesis and deuterogenesis, and so on). These concepts have
already been refuted in so far as amphibians are concerned by Vogt (1929)
and Bytel (1931), and they can similarly be dismissed as inapplicable
to teleosts. Gastrulation is a unitary process during which universal
morphogenetic movements bring the organ-forming territories of the head
and trunk, and doubtless of the tail, into position. Gastrulation
cannot be broken down into cephalogenetic and notogenetic phases; there
are no undifferentiated centres which form all the organs of a given
segment of the body.
The difficulties inherent in the concept of localized cell proliferation have already been pointed out. Never has such a process been
demonstrated to occur. Once again let us state, it is not enough to
discover a profusion of cells in mitosis; the newly formed cells must
be shown to increase in volume. And even if this were to be demonstrated
•
•
305
in other vertebrates, the fact that the mechanism does not play a role
in teleostean gastrulation means that it is not essential to gastrulation.
It could not be more than a secondary mechanism dependent on a more
constant process.
SUMMARY
1. In teleosts,.there are no pregastrulean movements on the surface
of the blastula, even though there is sometimes very active cellular
migration into the disk.
2. The vital staining method provided a means of delimiting the
areas occupied by the major presumptive organs at the beginning of
gastrulation. The arrangement of these areas (annular marginal zone,
chordamesoblastic zone and neural crescent) shows great similarity to the
pattern seen in anurean amphibians.
•
3. The organ-forming territories are brought into position by mass
movements of cells. These morphogenetic movements display the same
components as those observed in amphibians: invagination, convergence,
extension, epiboly, and ventral divergence to a lesser exèent.
4. The endoblast is formed by invagination of the superficial layers
(including the covering layer) of the dorsal marginal zone.
5. Neither delamination nor concrescence is involved in teleostean
grastrulation.
6. Counts of the number of cells in mitosis together with measurements of cell size in the various territories of the gastrula show that
cell proliferation is not involved in teleostean gastrulation.
7. The terminal node is not a growth centre but a crossroads of
morphogenetic movements.
8. There is no potential archentéron in teleosts that could be
considered the homologue of the archenteron of other anamniotes.
•
e
,
306
9. In the trout, no median fissure in the endoblastic layer can
be detected.
10. As in Urodelae, the hypochord forms through fusion beneath
the notochord of tissue moving inward from the internal edges of the
somites.
11. In the trout, the organs are brought into position very
gradually from anterior to posterior. While the organs are differentiating
anteriorly, gastrulation is continuing at the terminal node. This
chronological dissociation is a result of the large yolk of the Salmonidae.
It is not observed in most other teleosts, which have smaller eggs.
Gastrulation in these species bears a striking resemblance to amphibian
gastrulation.
12. The chordamesoblast must be considered to be of peristomial
origin in all chordates.
13. The archenteron is not always present and when it is, it is
a transient mosaic of disparate organs which cannot serve to define the
gastrula. This autilor is therefore opposed to the theory of the
gastrula-gastreae.
_
14. The blastopore is always present, not as a "moUth" leading into
the archenteron but as the orifice through which cells can migrate into
the disk.
•
15. Gastrulation must be defined dynamically.
is characterized by morphogenetic movements alone.
In vertebrates it
16. Centres of cell proliferation have never been demonstrated to
exist during gastrulation. Even if they were to be demonstrated in some
groups, their absence in teleosts means that they cannot be considered
an essential component of gastrulation.