The Early Development o f A s t r o p e c t e n irr e g u l a r i s , with Remarks on Duplicity in
Echinoderm Larvae.
By
H. G. Newth, A.R.C.Sc, D.I.C.,
L e c t u r e r i n E m b r y o l o g y , U n i v e r s i t y of B i r m i n g h a m .
W i t h Plates 40 a n d 41 a n d 2 Text-figures.
CONTENTS.
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INTRODUCTION
MATERIAL ANDMETHODS
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DESCRIPTION
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(a) S y m m e t r i c a l L a r v a e .
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(l)Twin Pore-Canal
(2) T w i n H y d r o c o e l a n d E n a
(6) T w i n L a r v a e
(c) A s y m m e t r y of t h e N o r m a l L a r v a .
(d) A n H y p o t h e s i s t o A c c o u n t f o r L a r v a l
(e) F a c t o r s i n G r o w t h
(/) C o n d i t i o n s of E a r l y D e v e l o p m e n t .
SUMMARY .
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Fertilization a n d Cleavage
Gastrulation
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Gastrula and Bipinnaria
Abnormal Larvae
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DISCUSSION
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529
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ntiomorphy
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Duplicity
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LITERATURE REFERENCES
EXPLANATION o rPLATES
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INTRODUCTION.
THE interesting, but fragmentary, accounts that we possess
of the metamorphosis of starfishes belonging to the families
Astropectinidae and Luidiidae (M. and C. Delap, 4 ; Mortensen,
21), make it very desirable that the complete development
520
H. G. NBWTH
of members of this group should be studied. Accordingly,
during a two-months' stay at the Plymouth Laboratory in the
summer of 1921, and again in 1923, in the intervals of other,
experimental work, I made repeated efforts to fertilize the eggs
of A s t r o p e c t e n i r r e g u l a r i s . In each year only one of
these attempts was successful. A plentiful supply of animals
was obtainable, but their gonads were small, and spermatozoa
removed from apparently ripe males were immobile when
suspended in sea-water—nor could they be made to function
by the usual device of altering the alkalinity of the water.
Small, but apparently healthy, cultures of larvae resulted from
the successful fertilizations. The larvae of 1923 perished from
unknown causes when only five days old, their early development having conformed exactly to that of the larvae of 1921,
and enabled me to make a few additional observations. The
following account is therefore based chiefly upon the earlier
culture, which, though it disappointed my hopes of witnessing
a complete development, furnished material of some general
morphological interest, and made it possible for the first time
to identify the early Bipinnaria of the present species.
MATERIAL AND METHODS.
The parents of the successful culture were collected on July 21,
1921, on the Eame-Eddystone trawling ground. Eggs were
obtained by gently teasing out the ovaries in filtered sea-water.
Fertilizations were made in glass finger-bowls, and in these the
larvae were kept until they had assumed the typical Bipinnaria
form and were feeding actively, a few drops of a culture of the
diatom N i t z s c h i a being supplied as food. They were later
transferred to a large bell-jar with a mechanical plunger, and
similarly fed. Freshly filtered water from outside the Plymouth
breakwater was used throughout, its alkalinity being found
to be practically unaffected by filtration. The temperature
was kept almost constant at about 16° C. My friend, Mr. E.
Ford, Naturalist at the Laboratory, very kindly took charge
of the culture when I left Plymouth, and the final sample was
preserved by him.
DEVELOPMENT OF ASTROPECTEN
521
The number of eggs fertilized was small, and fewer than
one hundred attained the early Bipinnaria stage—a number
which dwindled, owing to the usual causes of mortality, till
finally, on August 15, there remained only two individuals,
constituting the last stage here described.
The- figures are of animals preserved in Bouin's aqueous
picro-formol-acetic fixative, and in the case of whole larvae,
drawn while in alcohol, in which they suffer no distortion,
details being verified later in the stained and cleared preparation. Sections were cut in collodion and wax.
I.
DESCRIPTION.
1. F e r t i l i z a t i o n a n d C l e a v a g e .
F i r s t Day.—The eggs of the parent femalo, when removed
from the ovary, still showed, for the most part, large, vesicular
nuclei, indicating that the first maturation division had not
yet begun, but in a few (less than 10 per cent.) this stage had
been passed. Only a very small proportion of the spermatozoa
taken from the parent male were active in ' outside ' sea-water.
Eggs and sperm were set aside separately in a cool place, and
after about two hours it was estimated that between 10 per
cent, and 20 per cent, of the eggs had now undergone thenfirst maturation division. The average diameter of the mature
egg is 0-21 mm.
No change in motility of the sperm had occurred during the
ripening of the eggs, and the addition of alkali was without
effect. For insemination unmodified sperm-suspension was
used ; and at the end of five minutes' fertilization membranes
had been formed by practically all those eggs in which the
germinal vesicle had faded. The membrane, when fully
separated, stood out from the surface of the egg at a distance
equal to about one-tenth of its diameter, and carried with it
the first polar body. The second polar body was seen later to
be formed within the membrane.
The following observation may be recorded here, in view of
its possible bearing upon the second subject of this paper.
NO. 275
Mm
522'
H. G. NBWTH
A single batch of eggs was left unfertilized overnight and
inseminated next day. No further maturation took place, but
the ripe eggs segmented abnormally and none reached the blastula stage. Cleavage resulted in a wide separation of the blastomeres, reminiscent of the effect of ' calcium-free' sea-water.1
Observations of the normal cleavage of the egg were not made
in 1921, but in 1923 it was seen that after four or five regular
and synchronous divisions segmentation became irregular and
gave rise to an almost solid rnorula.
Nine hours after fertilization the blastula stage had been
reached. The embryo at this stage is of the wrinkled type
already described by other authors for various Asteroids,2 and
by myself for two species of Cu cum a r i a (22).
Six hours later the blastulae were still in their membranes—
which they now completely filled—and showed no sign of
ciliation or of invagination. The mean diameter of ten of them
was 0-238 mm. The embryos being fairly transparent, it could
now be seen that the blastula-wall was definitely epithelial,
and that the sulci indenting its surface had become shortened,
many of them appearing as pits rather than grooves. Fig. 1,
PI. 40, exhibits the appearance of an embryo of this stage in
section. It is noteworthy that the nuclei of its cells are—
almost without exception—situated nearer to the morphologically outer than to the inner surface of the blastula-wall—
a fact demonstrable, of course, only by reading the whole series
of sections.
'• I have since (1924) witnessed a similar spacing of the blastomeres in
certain cultures of A s t e r i a s r u b e n s . The later development of these
was perfectly normal, but the blastomeres were less widely separated than
in A s t r o p c c t e n .
a
It has several times been suggested to me that this type of blastula
is due to the abnormal conditions of laboratory culture. Since writing
the above I have been able to induce its formation experimentally in
E c h i n u s and A s t e r i a s , neither of which animals normally passes
through such a stage; and in both cases the experimental conditions
were totally unlike those of the A s t r o p e c t e n culture. It is quite
certain that a wrinkled blastula is normal in the case of A s t e r i n a
g i b b o s a—where it has never been described—for I have collected
embryos in this stage from the shore.
DEVELOPMENT OF ASTROPECTKN
523
2. G a s t r u l a t i o n .
Second Day.—Gastrulation was beginning at the end of
another seven hours, and the embryos, now ciliated and free
from their fertilization membranes, were rotating sluggishly
near the bottom of the bowl containing them.
Little or no smoothing-out of the folded blastula-wall precedes
gastrulation in the majority of larvae, except over the small
circular area that is actually invaginated. The folds are slowly
effaced during the formation of the archenteron. In the earliest
stages of this process the embryo is still approximately spherical,
and the archenteron is easily to be distinguished from other
re-entrant parts of the blastula-wall as being roughly rectangular in longitudinal section, while they are irregularly
rounded or pointed. Thus, the archenteron does not normally
arise in A s t r o p e c t e n as in S o l a s t e r e n d e c a (Gemmill,
6)—by the deepening of a pre-existing fold. Sections show that
the cell-nuclei are now situated at the inner (or blastocoelic)
ends of the cells, and that this is true of the whole integument.
Eapid general growth of the embryo, its elongation along
the archenteric axis, and an increase in its transparency due to
attenuation of the ectoderm, now begin, and continue throughout gastrulation. The egress of the infolded cells and their
incorporation into the general ectoderm go hand-in-hand with
these changes ; but this process is occasionally incomplete
even in the fully formed gastrula—which may thus retain
vestiges of the sulci which look like supplementary archentera.
These disappear later, leaving no trace.
The above observations have, in my opinion, their adverse
relevance to certain attempted explanations of gastrulation
that have been put forward. In the first place, they make
finally untenable a view still occasionally advanced, in spite
of other evidence against it: that reduced fluid pressure within
the blastula determines the imagination of its wall. If that
were true, we should be confronted in the case of A s t r o p e c t e n with a mechanical paradox: on the one hand,
evagination of re-entrant folds, and increase in volume of the
M m 2
524
H. G. NBWTH
embryo ; on the other hand, the concurrent ingrowth of the
archenteron. The first process, ex h y p o t h e s i , demands
an increase of internal pressure ; the second, a diminution.
Ehumbler's (26) ingenious interpretation makes invagination depend upon (a) absorption of fluid by the inner ends
of the cells concerned, (b) their consequent assumption of
a pyramidal shape, the base of the pyramid being inwards,
and (c) the lateral pressure of neighbouring cells, due to interstitial multiplication. Here, again, the gastrulation of A s t r o p e c t e n presents a difficulty. The invaginating area, it is
true, shows the usual pyramidal cells—but so, also, in many
cases, do the zones of evagination (fig. 2, PI. 40). Assheton's
criticism that the observed cell-form should be regarded as
a mechanical result—rather than a cause—of the infolding is
thus strengthened.
Assheton's own theory of gastrulation (1) was developed in
connexion "with the regular and symmetrical process seen in
A m p h i o x u s , and was supported by the behaviour of models.
It is quite inapplicable to A s t r o p e c t e n . The theory,
briefly stated, is that there exists between contiguous cells
a specific attraction acting from the neighbourhood of their
nuclei; and that a convex, free epithelium of such cells
(e.g. a blastula), in which, over a small area, the cell-nuclei
have migrated towards the convex outer surface, will tend to
invaginate the area in question. An obvious corollary—not
stated by Asshefcon—is the evagination, or smoothing-out, of
any re-entrant portion of a curved epithelium in which the
converse arrangement of nuclei is found. The egg of A s t r o p e c t e n furnishes, plainly, the ideal test of this hypothesis.
It will be sufficient to repeat what has already been said :
that in this animal the nuclei of the blastula are all nearer to
the outer than to the inner surface of the epithelial wall,
whilst the nuclei of the gastrula—whatever the immediate
positions or movements of the cells to which they belong—
are all near the inner, blastocoelic surface. The sections shown
in figs. 1 and 2, PI. 40, though not specially chosen to illustrate
the above points, exhibit sufficiently well the relations of the
DEVELOPMENT OF ASTROPECTEN
525
nuclei in blastula and gastrula ; and in each of these will he
seen folds of ectoderm in the last stage of evagination, whose
cells have the appearance—on Ehumbler's hypothesis—of
undergoing the diametrically opposite process.
3. G a s t r u l a a n d B i p i n n a r i a .
T h i r d Day.—There was great discrepancy in the rate of
early development, so that the next sample preserved contained
both late gastrulae, in which the coelom was just appearing,
and also larvae in which the Bipinnaria form was recognizable
(figs. 3-5, PI. 40).
The gastrula differs from those of A s t e r i a s and P o r a n i a
(Gemmill, 7 and 9) in that the archenteron, when fully
invaginated, reaches appreciably less than half-way towards
the anterior end, and in this way determines the characteristically bulky preoral lobe of the young Bipinnaria. The primary
coelomic vesicle arises, as in A s t e r i a s, from the expanded
tip of the archenteron, and in the living animal is a rather more
voluminous sac than is shown in fig. 3, PI. 40, where the result
of its partial collapse through fixation is seen. Wings of this
thin-walled expansion grow back on either side to form the
enterocoel pouches, which are completely separate from one
another save where they join the gut. The left-hand pouch is,
from the first, larger than the right.
An extensive but shallow in-sinking of the future ventral
surface forms the circumoral field, and initiates the curve of
the alimentary canal. In the middle of this depression the
stomodaeum appears as a small and quite definitely circumscribed invagination of the ectoderm, which comes into contact
with a blunt projection of the gut, and so remains till the endodermal oesophagus is constricted off from the stomach. Perforation of the mouth then occurs, and about the same time
the enterocoels lose their connexion with the gut. Soon after
this the left enterocoel acquires its communication with the
exterior through pore-canal and hydropore (fig. 5, PI. 40),
and faint indications of the ciliated band can be seen. Whether
526
H. 6. NEWTH
tan Auricularia stage occurs I cannot say with certainty. The
length of the larva is now about 0-4 to 0-5 mm.
S i x t h and E i g h t h Days.—The larvae were now
healthy, active Bipinnariae about 0-6 mm. long, with welldeveloped gut and ciliated bands. Fig. 7, PI. 41, makes further
description unnecessary.
T w e n t y - f i f t h Day.—The two larvae which alone reached
this age were nearly identical in size and appearance, and there
is no reason to doubt that their external features represent
those distinctive of the species (figs. 8 and 9, PI. 41).
Mortensen's illustrated descriptions of the two Japanese
species, A s t r o p e c t e n s c o p a r i u s and A s t r o p e c t e n
p o l y a c a n t h u s , are the only accounts we possess of larvae
of the genus A s t r o p e c t e n ; and to the first of these the
three-weeks-old Bipinnaria of A s t r o p e c t e n i r r e g u l a r is
bears a close resemblance. No appreciable increase in size,
and no change in the coelom except its growth in length, had
occurred since the eighth day. The boundaries of the enterocoels were not sufficiently plain in the whole preparations to
be included in my camera drawings, though it could be seen
that connexion between the anterior coeloms had not been
established in the preoral lobe, and that the hydrocoel had
not yet appeared—facts that were confirmed by serial sections.
The ciliated processes are short and rounded. The preoral
band is arched forward over the stomodaeum—though less so
than in A s t r o p e c t e n s c o p a r i u s . The median preoral
process projects ventrally, and at its base the convex frontal
area is strongly narrowed without the formation of salient
preoral processes at that point. Of the remaining processes
the anterodorsals are the best developed ; the posterodorsals,
posterolaterals, and postorals are small, though clearly defined.
The length of the larva when preserved is now about 0-7 mm.
The Bipinnaria of A s t r o p e c t e n s c o p a r i u s , as figured
by Mortensen, shows practically no growth, and but little
increase in the complexity of its processes, between the ninth
day and a stage of apparently advanced metamorphosis. It
is probable, therefore, that though my latest stage is still far
DEVELOPMENT OF ASTROPECTBN
527
from metamorphosis the description here given will suffice
for the diagnosis of the larva of A s t r o p e c t e n i r r e g u laris—at least among the Bipinnariae of British seas.
4. A b n o r m a l L a r v a e .
The commonest abnormality found in my cultures was the
presence of a hydropore and pore-canal on the right side as
well as on the left. This condition occurred in about onethird of the larvae examined, and is in no way exceptional,
since it has been noticed in all Asteroids that have been experimentally reared in sufficient numbers. No observations were
made of the persistence of this character, but it was still
present in two otherwise normal Bipinnariae examined on
the eighth day.
A very different kind—or degree—of abnormality is illustrated by the two individuals shown in fig. 6, PI. 40, and figs. 10,
11, PI. 41.
Contemporary with the normal larvae shown in figs. 3-5,
PI. 40, and discovered in the same fixed sample which contained them, were a number showing slight abnormalities of
development, mostly affecting the tip of the archenteron, and,
with one exception, not of a sufficiently definite nature to
justify a description here. This exception was in the case of
the animal depicted in fig. 6, PI. 40. The blind end of its
archenteron is expanded in the usual way to form a flattened
sac, but from this sac two small oesophageal rudiments
project, each met by a well-marked stomodaeum at the bottom
of a corresponding circumoral field. The edge of the primary
coelomic vesicle is irregular, but two larger projections from
it probably represent the first appearance of a pair of enterocoels—both dorsolateral in position. There is no indication of
hydropore or pore-canal; but the circumoral ciliated band is
visible, faintly indicated on the ventral surface just in front
of the anus, and seen on either side in optical section as a
thickening of the ectoderm. The orientation of the animal as
a whole is thus made certain.
The older abnormal larva (figs. 10 and 11, PI. 41)—a Bipin-
528
H. G. NEWTH
naria nine days old—was seen alive, but had unfortunately
escaped notice until I made a final survey of my cultures before
leaving Plymouth. When discovered it was active and
apparently healthy. It possesses two fully formed gullets,
a single stomach and intestine, but only one pair of enterocoel
pouches, each with a pore-canal and hydropore. The ciliated
band is in four parts : a typical preoral band lies in front of
TEXT-FIG. 1.
Diagram of a twin Bipinnaria (hypothetical) in the Auricularia stage,
to illustrate the probable homologies of the ciliated bands in the
animal shown in figs. 10 and 11, PL 41. P.O., postoral band,
median loop completed by dotted lines.
each stomodaeum ; dorsal and posterior to these is the postoral band, topographically normal; while, in addition, a
ciliated loop (m.l.) lies midway between the two frontal areas.
This median loop appears at first to be a third preoral band,
without a corresponding stomodaeum. I believe, on the
contrary, that it is a portion of the postoral band left isolated
by reason of the incompleteness of the twinning. Text-fig. 1
illustrates this interpretation by showing a hypothetical
monster in which the duplicity, both of ciliated bands and of
DEVELOPMENT OF ASTEOPBCTEN
529
alimentary canal, has been carried almost to the point of
separating the constituent twins, A and B.
II.
DISCUSSION.
In the absence of experimental analysis it would have been
presumptuous to have offered any explanation of the two
abnormal larvae described above—had they stood alone.
It happens, on the contrary, that they range themselves quite
naturally in series with a number of similar monstrosities
recorded elsewhere. A consideration of certain properties of
this series has led me to re-examine rather closely the current
explanations of duplicity in Echinoderm larvae, and, as a
result, to adopt a view that differs, in some respects, from
them all. Since my provisional hypothesis is one that may
prove fairly easy to test by experiment, I have thought it
would not be premature to set it forth here, together with
a brief commentary on some of the relevant observations and
opinions of previous workers.
Double monsters among Echinoderm larvae may be roughly
divided into two classes : one in which the duplicity affects
larval structures or the larva as a whole, and one in which it
affects the coelom and the rudiments of adult organs, leaving
the larval form apparently untouched. Examples of the first
class, so far as I can find, have only once before been described
(Gemmill, 8), and these I shall consider last.
Duplicity in the second class of larvae is of a very special
kind, and consists in the symmetrical repetition, on the right
side of the organism, of structures normally peculiar to the
left. It is not surprising, therefore, that those who have
imagined a bilaterally symmetrical ancestor for the group
should have hailed these variations of development as a
confirmation of their views. With the characters of this
ancestor I shall not be concerned here, except in so far as they
imply a fundamental—but ' latent'—bilateral symmetry in
the coelomic organs of normal Echinoderm larvae at the
present day. It is a part of my thesis that this latent symmetry
does not, in fact, exist.
530
H. G. NEWTH
(a) S y m m e t r i c a l L a r v a e .
(1) Twin Pore-canal.—The right enterocoel, as well as
the left, develops a pore-canal and hydropore. This condition
has been observed in varying proportions of the larvae of many
Asteroids, and in certain Ophioplutei and Echinoplutei. The
following are some examples of its incidence among experimentally reared Bipinnariae :
Species.
Author.
Gemmill (7)
Gemraill (7)
[iicidence.
10 per cent.
' At least 70 per cent, in certain cultures '.
Mortensen (20) ' About 50 per cent.'
Asterias glacialis
Field (5)
Asterias vulgaris
' Considerable numbers '.
Gemmill (9)
30-40 per cent, perfectly
Porania pulvillus
bilateral, ' only about 25
per cent, showed no trace '.
Astropecten irregu- Newth (present ' About one-third of the
t>at>er)
laris
larvae '.
Asterias rubens
Asterias glacialis
All these records are of larvae reared in the laboratory from
artificially fertilized eggs ; but it should be added that Gemmill (7) reports an incidence of 5 per cent, among larvae of
A s t e r i a s r u b e n s ' spawned and fertilized naturally in the
tanks ' (the conditions of cleavage and early development are
not stated). Apart from various less striking references to the
occurrence of this variation to be found in the literature of
experimental zoology, there are two cases that have been
considered of special morphological or phylogenetic importance.
A s t e r i a s v u l g a r i s (Field, 5) calls for special mention
because it is quoted as a case in which the symmetrical development of two hydropores has been demonstrated to be a normal,
though transitory, feature of development. This view of Field's
results, though persistently repeated, is quite erroneous. I
think that a careful collation of his statements should convince
any reader that only a comparatively small number of the larvae
observed by him actually possessed two hydropores, and that
his conclusion regarding the ' normal' occurrence of the
DEVELOPMENT OF ASTKOPECTEN
531
character is nothing more than a rather hazardous inference
from the facts recorded. It is certainly of great interest, however, that several larvae among the twins studied by Field
were obtained from the plankton, for this record—made more
than thirty years ago—still stands almost alone, in spite of
the large number of pelagic Bipinnariae that have since been
examined.
The second case is that of the larva of the Clypeastroid seaurchin, M e 11 i t a p e n t a p o r a (M. t e s t u d i n a t a), studied
by Grave (11 and 12). Here the bilateral symmetry is said
to be completed in a different way : ' . . . two pore-canals are
of constant occurrence. . . . Communicating with the single
median dorsal pore two well-developed canals are seen, one
joining the left, the other joining the right anterior enterocoel.
The right canal is usually slightly smaller than the left but
it is never entirely wanting. It persists in the adult as a small
closed vesicle in the region of the ampulla which receives the
internal opening, or openings, of the madreporite and the
terminal opening of the stone-canal' (Grave, 11). Unfortunately Grave does not describe the way in which this apparently symmetrical arrangement comes about. There would
appear to be two possibilities : (a) the already separated enterocoels may each send out a canal which meets its fellow at the
common hydropore ; or (b) the primary coelomic vesicle may
acquire its communication with the exterior while it is as yet
undivided, in which case the right' pore-canal' would represent
the remains of the original connexion between the enterocoels.
Against the former alternative is a certain a p r i o r i improbability, and also the fact that no comparable process has
hitherto been observed elsewhere; while in favour of the
latter alternative are the observations mentioned in the next
paragraph. We have also Grave's own statement that the
right ' pore-canal' persists as what we are fairly safe in calling
the madreporic vesicle of the adult—a closed vesicle in the
region of the ampulla of the stone-canal. Unless, then, we are
prepared to draw the fantastic conclusion that the madreporic
vesicle of Bchinoderms generally is a vestigial right poro-
532
H. G. NBWTH
canal, the case for the complete bilateral symmetry of the
Mel lit a Pluteus falls to the ground.
Gemmill (9) states of P o r a n i a that the coelom generally
separates as a single dorsal sac which at once divides right and
left. It seems, further, that the appearance of the pore-canal, or
pore-canals, normally follows immediately upon this separation
of the enterocoels—so that there are three processes occurring
in quick succession. Occasionally, however, the enterocoels
retained their dorsal connexion till even the third week (9,
p. 34). The early stages of such larvae were not observed, but
I have myself recently witnessed, as a rare abnormality in
the development of A s t e r i a s r u b e n s , the acquisition of
a pore-canal by the undivided primary vesicle, the canal being
later appropriated by the left enterocoel; and a very similar
observation is recorded by Gemmill (7) for the same species.
There is in these cases what may be called a dislocation in the
sequence of development, whereby the division of the primary
vesicle is delayed. It is not unreasonable to suppose that such
a dislocation may also occur in the reverse sense, the primary
vesicle dividing too soon instead of too late ; so that any
processes normally taking place in it as a prelude to its
division would be anticipated. The consequences and possible
causes of such precocity will be dealt with below ; but it
may be recalled here as a significant fact that in Porania—•
in which the events connected with the early differentiation
of the eoelom occur so rapidly—there is an extraordinarily high
percentage of pore-canal twins. Certain abnormal E c h i n u s
larvae that have been recorded as having dorsally confluent
axial sinuses sharing a single pore-canal may be mentioned
in this connexion, though their early stages have not been
observed (Ohshima, 24, cases 5 and 7).
No satisfactory explanation of twin pore-canal has been
offered. Field saw in its occurrence the further expression of
an underlying, but much obscured, bilateral symmetry, due to
descent from an ancestor in which that symmetry was complete. Other authors have since accepted the same view,
MacBride asserting that it is ' the key to the understanding
DEVELOPMENT OF ASTROPECTEN
533
of Echinoderm development' (16, p. 466). G-emmill, however,
on account of the widely varying frequency of the character,
does not ' ascribe the incidence of double hydropore directly
to ancestral causes '—but to Homoeosis.
(2) Twin H y d r o c o e l a n d B n a n t i o m o r p h y . — T h e
right side of the larva, as well as the left, develops—in varying
degrees—a hydrocoel and its associated structures ; or the
right side alone develops them, in which case the whole system
becomes a mirror-image, or enantiomorph, of the normal.
The propriety of including the latter condition in the category
of symmetrical larvae will appear later.
Among Asteroids, Ophiuroids, and Echinoids, individual
larvae with two symmetrical hydrocoels have been occasionally
recorded for many years past; but only recently have they
been obtained in large numbers, and studied in such detail as
to warrant any conjecture with regard to their significance
or mode of origin. In only one case of this kind is a vestigial
right hydrocoel described as being a constant feature of normal
development ; and since great importance has been attached
to this case I shall consider it first and in some detail.
In his paper on O p h i o t h r i x f r a g i l i s (14), MacBride
describes and figures a single pluteus—taken from the plankton
—in which there were two well-doveloped hydrocoels. He
describes also, as a normal feature of development both in the
sea and in artificial culture, the separation of a vesicle of
variable size and appearance from the right anterior coelom.
This structure he considers, on account of its place of origin,
to be the antimere of the hydrocoel. Such a view should,
I think, be accepted with extreme caution ; because when the
account of O p h i o t h r i x was published its author still believed
that a somewhat similar vesicle, formed from the right anterior
coelom in A s t e r i n a and E c h i n u s , was a vestigial right
hydrocoel. In Asteroids and Echinoids this structure has been
proved subsequently by Gemmill (7), and by MacBride himself
(17), to be the madreporic vesicle, an organ quite distinct,
which may be present in the same abnormal larva together
with a well-developed right hydrocoel. When it is added that
534
H. G. NEWTH
during the metamorphosis of O p h i o t h r i x the sac in question
comes to lie close to the hydropore, and that ' sometimes a
projection of its inner wall is noticeable similar to that which
gives it acrescentic form in A s t e r i n a g i b b o s a ' (MacBride,
14), the strong probability must be admitted that it is here,
as in Asteroids and Echinoids, not the right hydrocoel but the
madreporic vesicle. In a later publication (16) MacBride,
after admitting the distinction between hydrocoel and madreporic vesicle in Asteroids (p. 467), reiterates his opinion that
O p h i o t h r i x has a right hydrocoel (pp. 491 and 492), and
then states : ' A madreporic vesicle is formed, apparently
in the same way as in Asteroidea.' This may mean that an
additional sac is formed in O p h i o t h r i x ; but I can find no
reference to it elsewhere. If it does not exist there seems no
possibility of reconciling the three statements made, except
by supposing the madreporic vesicle to possess different honiologies in different groups of Echinoderms. I shall prefer to
assume that an error of interpretation has been made and
inadvertently perpetuated.
Excellent accounts of twin hydrocoel larvae in Asteroids have
recently been given by Gemmill (10), in Echinoids, by MacBride (15 and 17), and Ohshima (23 and 24). For a full description of cases and for references to previous observations the
reader must consult these authors : I shall be able only to
summarize the large amount of material available.
(Temmill's account is based upon more than sixty twin
hydrocoel larvae of A s t e r i a s r u b e n s found in his cultures
during 1912 and 1913. Many of these showed almost complete
symmetry of coelomic organs up to an advanced stage. The
presence of two hydrocoels and two hypogastric coeloms
involved the absence of an epigastric coelom s e n s u s t r i c t o ,
but a single, somewhat defective aboral complex of organs
arose dorsally—in part over the dorsal wings of the hypogastric coeloms, in part over a pseudocoelomic space developed
in the dorsal mesentery. Six or seven individuals actually
metamorphosed and lived for a short time after. In the
normal development of A s t e r i a s asymmetry of the posterior
DEVELOPMENT OF ASTROPECTEN
585
coeloms declares itself, some clays before the appearance of the
hydrocoel, by the outgrowth of a ventral horn of the left
posterior coelom and its fusion with the right middle coelom,
a process with no counterpart on the right side ; and it is
interesting to find that in these twin hydrocoel larvae the first
sign of duplicity was the failure of this fusion to occur (10,
p. 54). Gemmill ascribes this failure, in some cases, to malnutrition and a consequent inability of the ventral horn to
extend; in other cases—the majority—he supposes it to be due
to excessive nutrition and a consequent enlargement of the
stomach, which became a mechanical obstacle to the oxitgrowing ventral horn. In either case the right middle eoelom,
thus isolated, developed a hydrocoel if subsequent conditions
were favourable. One fact, recorded without comment, here
seems to me to be of capital importance : that the ventral
horn of the left posterior coelom, though it failed to reach its
normal destination, united with a similar ventral horn of the
right posterior coelom ; and apparently this occurred before
the formation of the hydrocoels. In other words, the symmetry
of the coelom was complete before the hydrocoels appeared.
It is difficult to see how either excess or defect of food could
account for this.
In one respect all the twin hydrocoel larvae of A s t e r i a s
were asymmetrical: not one had a right pore-canal or hydropore. These organs, however, when developed, usually atrophy
long before a hydrocoel appears. It does not follow, therefore,
that the two symmetrical variations are independent in origin.
That conclusion would be valid only if it were proved that there
was no difference between the incidence of twin pore-canal in the
early stages of normal larvae and its incidence in similar stages
of twin hydrocoel larvae from the same batch of eggs.
Gemmill's double larvae were all developed from artificially
fertilized eggs ; and among several hundred plankton larvae
that were surveyed not a single twin hydrocoel was found.
From all these facts he concludes that laboratory conditions
such as (a.) hurried maturation, &c, of the eggs, and (b) irregularities of larval nutrition, disturb the normal course of develop-
536
H. G. NBWTH
ment; but that such disturbance could not ' supply guidance
in the production of double hydrocoel ' and serves only as an
opening for the play of those agencies—atavism and homoeosis
-which produce the specific effect.
MacBride (17, 18, 19), working with E c h i n u s m i l i a r i s ,
claimed to have produced twin hydrocoel experimentally by
the transference of very young Plutei to sea-water of increased
salinity. The incidence was 2 per cent, and ' at least 5 per cent.'
in two ' treated ' cultures, while only one symmetrical individual was found among hundreds of larvae in the controls.
MacBride explained his results by supposing that in the
normal right anterior coelom there resides a ' latent power '
to develop a hydrocoel, and that this was awakened by the
stimulus of the hypertonic water. Other symmetrically
developed structures were due to the emanation of hormones
from the growing hydrocoel. Later, using the same methods
in the same laboratory, Ohshima (24) obtained more abnormal
larvae in his controls than in his treated cultures ; but in
this case the commonest abnormalities were enantiomorphs,
of which the incidence was more than 10 per cent, in about
1,400 larvae examined.
Twin hydrocoel in Echinoids does not differ in essential
features from the same variation in Asteroids : the right side
becomes in many cases an almost exact, but reversed, copy of
the left. Many differences occur, however, in the degree to
which the organs of the two sides are developed. A hydrocoel
may be absent altogether ; and in some cases where two were
present MacBride found pedicellariae (aboral structures of the
adult) also developed on both sides—a complication to which
I shall recur. Of twenty cases of twin hydrocoel reviewed by
Ohshima, two had pore-canal and hydropore on the right side
only, and nine showed twin pore-canal. The numbers are
too small to justify a positive inference, but the proportion of
twin pore-canal larvae is so large that correlation between
this condition and the presence of two hydrocoels is rendered
very probable.
The three main categories of abnormality just mentioned,
DEVELOPMENT OF ASTROPECTEN
537
(a) twin hydrocoel, (6) enantiomorphs, (c) absence of hydrocoel,
are all, according to Ohshima, due to the same initial cause—
arrest of development and consequent atrophy of the normal
left hydrocoel. This cause, acting alone, will produce larvae
devoid of hydrocoel (MacBride, 17), but in certain conditions,
Ohshima maintains, the right anterior coelom, relieved of the
normal inhibitory presence of a left hydrocoel, may have its own
latent capacity to form a hydrocoel aroused, and there will
result an enantiomorphic larva—or, if the left hydrocoel itself
recovers, a twin hydrocoel larva. Prom the rarity of abnormalities in the sea Ohshima concludes, with Gemmill, that
laboratory conditions are responsible, accidental occlusion of
the hydropore (e. g. by food-diatoms) being the main cause of
degeneration of the hydrocoel. Like Gemmill, too, he relies
on Homoeosis for the actual appearance of a right hydrocoel.
But whether the two authors use this word in the same sense
I am still unable to determine after repeatedly reading their
papers with great attention ; and I can heartily endorse the
criticism of MacBride (18 and 19) that to invoke such a principle
is to substitute a word for an explanation.
What is common to the interpretations of all three of these
workers is this : apart from their admission of possibly contributory, but indefinite, disturbances of early development,
they refer duplicity to agencies acting upon normal larvae at
a stage just before hydrocoel formation. No attempt, however,
has hitherto been made to eliminate conditions of early development that might be competent to cause duplicity ; and till
that is done the assumption that the anterior coeloms are in
any way normally equipotent cannot be said to rest on experimental evidence.
(b) Twin L a r v a e .
The larva as a whole is in some degree affected, as is evidenced
by the duplication of axial structures such as the alimentary
canal. Apart from the present paper I know of only one record
of the occurrence of this form of abnormality.
Gemmill (8) found a number of such larvae in a culture of
NO. 275
N n
538
H. G. NEWTH
1
L u i d i a s a r s i that had passed through their first stages
of development from the early blastula in transit from Plymouth
to Glasgow in thermos flasks. Eleven gastrulae and nine young
Bipinnariae are described. The gastrulae show various degrees
of duplicity of the archenteron, ranging from one in which that
structure is completely double from end to end, through
V-shaped, A-shaped, Y-shaped, A-shaped forms, to one,
finally, in which the blind end of the archenteron is only
slightly forked. The Bipinnariae correspond in their stage of
development to the A s t r o p e c t e n larva shown in figs. 10
and 11, PI. 41. They exhibit degrees of duplicity and orientations of twinning, similar to those met with in vertebrates, the
gut and ciliated bands being taken as criteria. The condition
of the enterocoels is extremely interesting. It can be summarized by saying that where the endodermal oesophagus has
been doubled there is an attempt to form two pairs of enterocoels, that this has completely succeeded in all except two
cases, and that, of the four sacs present in any larva, the
morphologically left-hand one has a pore-canal. In the two
exceptional larvae—apparently owing to the exigencies of
space—three instead of four sacs have been formed, and here
it is the left and the middle sacs which have pore-canals
(8, figs. 19 and 21). The remaining double larva (fig. 16)
is one in which the endodermal oesophagus is not obviously
involved, though there are two stomodaea. It possesses a single
pair of sacs, apparently normal. In none of the larvae does
a free morphologically right enterocoel possess a pore-canal.
Gemmill attributes the formation of these monsters to the
shaking sustained by the culture on its long journey, the
consequent ' early partial separation of cells or of cell masses '
having caused more or less doubling of the area of invagination.
He definitely assimilates the appearances presented by the
Bipinnariae to those found in the twin monsters of Vertebrate
teratology, which are due to the appearance of ' two foci of
embryo formation ' ; but he states, just as definitely, that
1
Gemmill speaks of L, s a r s i , but identifies it with the species bred
by Mortensen (20), L. e i l i a r i s , which is much commoner at Plymouth.
DEVELOPMENT OF ASTROPECTEN
539
they are not comparable to ' double E c h i n u s rudiments . . .
described in detail by MacBride '.
Ohshima (23 and 24) has cast doubt on this interpretation,
and suggested that f u s i o n of individuals has occurred. Two or
three of the gastrulae and one of the Bipinnariae certainly look
in the figures as if they might have been formed in this way ;
but as to the remaining sixteen or seventeen larvae, it is scarcely
credible that random approximation should have led to union
in pairs only, and moreover that this union should have been
of such a sort that the anterior and posterior ends of the
conjugants always fused with their like.
The similarity of the L u i d i a twins to those of A s t r o pec t e n , described on p. 527 of this paper, is obvious. The
following facts may have a bearing on their occurrence. The
families to which the two species belong are considered to be
nearly related ; the eggs of the two species are very similar
in size, and somewhat large for animals with a pelagic larva
( A s t r o p e c t e n , d = 021 mm.; L u i d i a , 1 d = 0-215022 mm., i. e. they are about twice the volume of the egg of
A s t e r i a s r u b e n s , d = 0-16-019 mm.) ; both species pass
through a wrinkled blastula stage, the irregularity of which
may facilitate—if not cause—irregularities of gastrulation.
(c) A s y m m e t r y of t h e N o r m a l L a r v a .
I have already reviewed several of the more striking and
exceptional cases in which bilateral symmetry of coelomic
organs has been alleged to occur in normal development;
and I have suggested that the evidence for such symmetry
is insufficient. The evidence from more ordinary ontogenies
will be briefly examined.
In those Asteroids, Opbiuroids, and Echinoids which have
a pelagic larva, four coelomic sacs are formed by the roughly
symmetrical growth backwards of two horns of a primary
vesicle, arising from the tip of the archenteron; by the separation of these horns from their point of origin; and by their
1
Unpublished information for which I am indebted to Professor
GemmilFs kindness.
540
H. G. NEWTH
subsequent division into two on either side. In those members
of the same groups which have more yolky eggs the process
is often different, and may be manifestly asymmetrical.
In the only other group in which a typical pelagic larva occurs
—the Holothurioids—the process is always asymmetrical:
the primary vesicle does not divide right and left, but either
moves bodily to the left side or originally appears in that
position, where it gives rise to the hydrocoel, after segmenting
off the single rudiment of the posterior coeloms. The view that
the more symmetrical of these processes are also more fundamental is based on a series of phylogenetic assumptions which
cannot be tested by existing methods of research, and which
therefore need not be retailed here. There is, on the other
hand, some objective evidence for believing that even in the
more symmetrical larvae the coelom is from the first in reality
asymmetrical.
MacBride (14) early drew attention to a disparit}- in the rate
of development of the two sides, at a time preceding the
formation of the hydrocoel, in A s t e r in a, E c h i n u s , and
O p h i o t h r i x , and stated that it foreshadowed ' that predominance of the left side which plays such an important part
in the metamorphosis '. I have already referred to the early
asymmetry of the posterior coeloms of A s t e r i a s (p. 53o).
As the result of experimental work, Eunnstrom (29) came
to the conclusion that asymmetry was determined much earlier,
and observation of normal development bears this out. In
the present paper I have stated that the left enterocoel of
A s t r o p e c t e n is initially larger than tho right ; Gemmill
says that in P o r a n i a (9) ' a s a rule', in A s t e r i a s (7)
' not infrequently at first', the left sac is slightly the larger ;
and an examination of the figures of early stages given by
various authors makes it probable that this is universal. Eunnstrom, indeed, claims in another paper (27) that the gastrula
of P a r e c h i n u s m.iliaris ( E c h i n u s m i l i a r i s ) already
shows asymmetry of the anterior end of the archenteron at
a stage when the coelom can hardly be said to be indicated :
' Der vorderste Teil des Urdarms ist asymmetrisch gebaut.'
DEVELOPMENT OF ASTROPECTEN
541
When it is borne in mind that these early stages have generally
been passed over with scant attention, such observations
acquire great significance.
To say that the asymmetry of a later stage is ' foreshadowed '
is surely to admit in metaphorical language that the organs
upon which that asymmetry depends are already determined:
that the dispositions which normally lead to their appearance
are present on one side and absent on the other. If it does not
mean this, what does it mean ? If it does, then it is idle to
speak of characters of the left side being normally ' latent'
on the right. For it is upon just such small adumbrations as
have been mentioned that—in the absence of experimental
analysis—we usually rely for information on the processes of
development.
(d) An H y p o t h e s i s to A c c o u n t for L a r v a l
Duplicity.
If the facts concerning abnormal Echinoderm larvae that
have now been reviewed are considered without phylogenetic
preconceptions, it will be almost impossible to avoid the conclusion that they are of the same kind as the facts of vertebrate
teratology. This similarity is, naturally, more striking in the
case of the twin Bipinnariae described by Gemmill and myself,
where the duplicity involves chiefly larval organs which are
bilaterally symmetrical; but the parallel is almost as complete
in cases of twin hydrocoel. If, for the sake of comparison, we
regard the larval organs as simply a trophic appendage to the
growing urchin or starfish, it is apparent at once that in normal
development the rudiment of the adult bears much the sanie
relation to the larva as—for instance—the embryo of a bird
bears to its yolk-mass and embryonic membranes. The analogy
might easily be elaborated. I need only point out here that
in both cases the system as a whole is asymmetrical with reference to its original median plane, and that in both cases the
developing adult possesses its own asymmetry of organization.
Let us now consider a fowl embryo of the third day exhibiting
the kind of twinning known as sternopagus—that is to say,
542
H. G. NEWTH
an H-shaped double monster in which the anterior and posterior
ends of each of the twins are free, and there is union in the
thoracic region, with the two hearts—or a common heart—in
the isthmus (cf. Eabaud, 25). Abnormalities of this kind are
by no means rare among vertebrates. The left-hand twin
is normal in its relations, while the right-hand twin is enantiomorphic—its head being twisted so as to lie with the right
side instead of the left turned towards the yolk, and its heart
showing s i t u s i n v e r s u s . It will be seen that the system
as a whole is here symmetrical about a plane, the asymmetry
of the one twin being balanced by the reversed asymmetry of
the other. This is precisely the state of affairs in the twin
hydrocoel larvae of Echinoderms.
Now there is good reason for believing that, in Vertebrates,
such forms of duplicity as I have mentioned are due to the
earlier partial fission of a region of active growth and differentiation—the edge of the blastopore or the primitive streak.
We know also that mechanical and other interference with the
segmenting egg, or physiological inhibition of the closing blastopore, can cause a doubling of the ' foci of embryo formation '.
Is there in Echinoderm development a comparable growthcentre to which subsequent duplicity of adult organs can be
referred ?
As regards the coelomic organs the answer, I think, must be
that there is such a region in the blind end of the archenteron
at the time of the formation of the coelornic rudiment, and it
is mainly in terms of the doubling of this region that I shall
now attempt to bring into line the several kinds of observed
duplicity.
I. If my previous criticism has been just, Ave may say that
in the normal larva the characters of the enterocoels are
already determined at their first appearance ; and, further,
we may assume that in the single vesicle from which they
spring, or in the tip of the archenteron, there occurs a segregation of their potentialities right and left. Such a supposition
is quite in accord with what is believed to take place in other
embryonic processes. The undivided primary vesicle can be
DEVELOPMENT OF ASTROPECTEN
543
described, then, as b i v a l e n t with regard to the potential
characters of the right and left enterocoels (Text-fig. 2, A).
II. The presence of two or more less widely separated areas
of invagination, during gastrulation, will lead to various degrees
of duplicity of the archenteron (if Gemmill's interpretation is
correct, the twin gastrulae observed by him are actual illustrations of this process). The extreme case is one where there are
two complete archentera side by side as the result of widely
separated invaginating areas ; and successive approximations
of these areas will lead to diminishing degrees of bifurcation
of the anterior end of a single archenteron. But this bifurcation, whatever its degree, must necessarily involve the coelomic
rudiment.
(1) Anterior doubling of the archenteron sufficient to affect
the fore end of the gut will at the same time lead to the formation either of two completely separate primary vesicles, or of
two which are in contact or joined side to side. Each will be
bivalent, and e a c h will attempt to produce two enterocoels—
an attempt that would be expected to succeed in proportion
to the divarication of the limbs of the forked gut:
(a) Complete success will result in the formation by the
larva of four enterocoels—two pairs of normal antimeres.
(b) Partial success will lead to the formation of three enterocoels, two lateral and one median, the latter equivalent to the
right enterocoel of the left-hand pair p l u s the left enterocoel
of the right-hand pair. This median sac will be bivalent,
and will possess the capacity of forming a pore-canal.
These relations are exhibited in Text-fig. 2, rows C and D.
The conditions there portrayed—excepting the middle term
of each,sequence—are not suppositional, but are those present
in actual larvae described by Gemmill (see p. 538). Now, the
twin Bipinnaria of A s t r o p e c t e n (figs. 10 and 11, PI. 41)
shows, besides anterior duplicity, a certain degree of back-toback twinning, which is revealed by the facing outwards of the
stomodaea and the appearance between them of part of the
postoral ciliated band. In other words, the dorsal region is
narrowed ; and this, on the assumptions made, will have the
544
H. G. NEWTH
same effect on the twin primary vesicles as that of a lesser
degree of bifurcation of the archenteron, i.e. the median
enterocoel will be suppressed, and the two sacs formed will be
TEXT-FIO. 2.
Diagram illustrating the relations of the coelom and gut in normal
and double Echinoderm larvae. Larvae shown in dorsal aspect;
stippling indicates normal potentialities of left enterocoel;
vertical rows are individual sequences; horizontal rows, equivalent
stages. A, N o r m a l L a r v a e : segregation of potentialities
undisturbed, single hydro pore. B, S y m m e t r i c a l L a r v a e :
segregation anticipated, precocious division of coelomic rudiment,
two hydropores. C and D, Twin L a r v a e . C, anterior
duplicity; archenteron forked; larval gut affected; three
enterocoels—middle one bivalent, with pore. D, parallel duplicity,
archenteron and all derived organs completely duplicated. (Lettering as in Pis. 40 and 41.)
bivalent as in the next case to be considered. The A s t r o p e c t e n larva is, thus, one term in a series of forms showing
diminishing duplicity of the alimentary canal and ciliated
DEVELOPMENT OF ASTEOPECTEN
545
bands, and at the same time—quite consistently—of the
coelom also. A final term in this series remains to be discussed.
(2) Anterior doubling of the archenteron so slight as not to
reach the oesophageal region will affect the coelomic rudiment
only ; and, on analogy with the appearances in twin L u i d i a
and A s t r o p e c t e n larvae, we may expect the external form
of the larva not to show—at first—any duplicity at all. The
doubling, in fact, may now be supposed to constitute, or
produce, the precocious division of an apparently single
primary coelomic vesicle whose lateral halves become the
enterocoels, as in a normal larva. Each enterocoel, however,
will be bivalent and capable of forming a pore-canal and, in
suitable conditions, a hydrocoel (Text-fig. 2, B). The particular
manifestations of this degree of duplicity will depend upon
later conditions. The pore-canal is an organ formed soon
after the separation of the enterocoels, and since it is of small
size its appearance is independent of the food-supply—in fact
it is formed even by unfed larvae. A right pore-canal would
thus be the first structural indication of duplicity in an outwardly normal larva. The hydrocoel, on the other hand, has
rightly been called by MacBride an ' expensive' structure
formed after the initial impetus of development has been
exhausted. The appearance of even one hydrocoel is dependent
on abundant food ; the development of two requires a superabundance. This is a partial explanation of the greater
frequency of twin pore-canal.
Duplicity of the archenteron has been assumed throughout
to be traceable to an early stage of invagination, and we may
therefore describe the duplicity of the coelomic organs, &c.
which arises in this way as being o b l i g a t o r y .
If, however, in the above scheme I have dealt with duplicity
particularly in terms of the archenteron and the coelom, it
has been for the sake of simplicity of exposition, and without
any intention to claim autonomy for these parts. In the case
of twin larvae the external structure is no less affected than
the internal; and there are indications that this is true of
twin hydrocoel larvae. It was urged by Grave (IS), as an argu-
546
H. G. NEWTH
ment against invoking atavism to account for a right hydrocoel,
that a right amniotic invagination was not possessed by the
ancestor, but nevertheless occurred in symmetrical larvae.
MacBride explained its presence by supposing that a stimulus
from the underlying hydrocoel activated the ectoderm. But
the same author has homologized the amnion of Echinoids
with a part of the larval stomodaeum delayed in its appearance—a view the correctness of which has since been made
almost certain by Mortensen's observations on P e r on el la
(21). If it is indeed correct, the appearance of an amnion on
the right side of a twin hydrocoel larva is not remarkable,
and no causes more recondite than those here suggested seem
to be called for to explain it. Both right hydrocoel and right
amnion can be referred to the twinning of median structures
—archenteron and stomodaeum—of which they are respectively
the derivatives.
(e) F a c t o r s in G r o w t h .
Hitherto I have designedly spoken of the ' potentialities' of
organ-rudiments because the term is non-committal with regard
to the nature of the processes of growth that are involved.
Both the facts and their attempted explanation may, however,
be expressed in terms of the metabolic gradients that are—
some of them certainly, others by inference—concerned in
development, without any modification of the fundamental
conception set forth.
Child (2) demonstrated the presence of a well-marked axial
susceptibility gradient in the unfertilized eggs, cleavage stages,
blastulae, gastrulae, and young Bipinnariae of A s t e r i a s
f o r b e s i i . The gradient axis coincided in all cases with the
morphological axis, the apical region being at the animal pole
of the egg, and at the anterior end of the gastrula and young
Bipinnaria. Unfortunately Child was unable to obtain later
larvae, but it is extremely significant to find that this initial
gradient in the Bipinnaria became less and less distinct as
development proceeded, and—at least as regards the ectoderm—
that it was finally reversed in a certain percentage of larvae,
DEVELOPMENT OP ASTROPEOTEN
547
the posterior part of the body showing a slightly higher susceptibility. An outline sketch of a larva in this stage shows
a young Bipinnaria in which the enterocoels and pore-canal
—though they are not figured—would be expected to be
well established.
Throughout gastrulation there is a centre of growth and
differentiation at the tip of the archenteron, which gives rise
ultimately to the coelom. Soon after this the essentially larval
characters are nearly all present as rudiments, the animal
begins to feed, and the susceptibility gradient disappears.
Henceforward the new growth is, in the main, associated with
the extension of the coelom backwards along the gut on either
side; and I suggest that the appearance of the reversed gradient
noted by Child is a manifestation of the fact that the advancing
tip of the enterocoel is a metabolic apex. It is at, or near to,
this apical region that the pore-canal is formed ; later the
whole apical region is cut off as the posterior coelom on either
side ; whereupon, on the left side, the physiological isolation
thus conferred upon the anterior coelom enables it to form a
new apical organ—the hydrocoel. Eunnstrbm (28) concludes
from his experiments on E c h i n u s that the difference between
the two asymmetrical sides of the larva is quantitative, and
consists in a difference of metabolic rate which appears early
in development. Indeed, accepting the evidence of normal,
left-sided development alone, we should suppose that from the
first separation of the enterocoels the gradient on the left
side was more strongly marked than that on the right; or,
in other words, that there was a secondary gradient from left to
right across the larval body. This gradient—ultimately the
axial gradient of the adult, with its dominant region on the
oral surface—probably originates in the undifferentiated
primary coelomic vesicle, and what I have called a segregation
of potentialities is here, as in other cases, the establishment of
a metabolic axis. If this is the case we have a possible secondary
cause of duplicity suggested to us. In seedling plants the
removal or inhibition of a growing tip will, in some cases, cause
the formation of paired axillary shoots. If these appear
548
H. G. NBWTH
simultaneously and grow initially at the same rate they may
both persist, but if one appears earlier or grows more rapidly
it inhibits the growth of the other (cf. Child, 3, p. 152). The
similarity of these relations to those between the two hydrocoels
of double Echinoid larvae (Ohshima, 24) is very striking, and
points to an essential identity of the growth-processes concerned.
In the next section I shall attempt to show that there are,
in fact, conditions in laboratory cultures which may mechanically damage or physiologically inhibit the tip of the archenteron, and so produce in a previously normal larva what may
be called Facultative Duplicity in contrast with the Obligatory
Duplicity already noticed. It is only necessary to add here that,
given the presence of either of these conditions in the early
larva, the relations of dominance and subordination between
regions of higher and lower metabolic rate will account for
many, if not all, of the discrepant facts mentioned in the
earlier part of this discussion. Among these are the transitory
nature of the right pore-canal in Asteroids, the production of
enantiomorphs in Echinoids (dominance of the right-hand
member of a pair of bivalent enterocoels), the development of
pedicellariae on both sides of twin hydrocoel Echinoplutei, and
finally the occurrence of an enantiomorphic Anrieularia—which
otherwise seems quite inexplicable (Ohshima, 24).
(/) C o n d i t i o n s of E a r l y D e v e l o p m e n t .
The unnatural conditions present in ordinary laboratory
cultures have not been left out of account by other authors,
but their importance has, I think, been greatly under-estimated.
To obtain eggs for fertilization it is often necessary to detach
them more or less forcibly from the ovary—by ' shredding out '
(Geinmill), shaking the ovary in a bag of bolting-silk in seawater, or some such process. Thus obtained, the eggs are
sometimes mature, and presumably ready for fertilization,
or they may mature after standing for an hour or two, during
which time they are exposed to influences that may affect
their polarity in a manner to which I shall refer below. Apart
from the chance of gross mechanical injury there is thus a
DEVELOPMENT OF ASTROPECTEN
549
possibility that, with reference to the establishment of the
axial gradient, eggs may be matured and fertilized precociously.
The effect of raised temperature upon development has, of
course, been studied in detail in a number of animals, but
generally as regards greater departures from the normal than
those occurring in carefully conducted fertilization experiments. Heat has a marked effect in producing abnormalities
of segmentation and gastrulation (including duplicity) in both
vertebrates and invertebrates, and it must be remembered that
even in the favourable conditions of the Plymouth Laboratory
it is impossible to maintain cultures at a temperature as low
by several degrees as that of the sea. Des Arts in the case of
C u c u m a r i a f r o n d o s a showed that small increases in
temperature can seriously interfere with the normal development of that animal.
The conditions of the eggs during cleavage are exceedingly
unnatural, and are such, moreover, as may easily entail drastic
internal changes. It is safe to say that in the majority of
Echinoderms, in which the eggs are shed freely in the sea and
remain unattached, they are subjected in a state of nature to
no external influence that could be d i r e c t i v e . Since they
are in suspension they are equally oxygenated on all sides,
and their orientation with reference to gravity is either constantly changing, or can attain its optimum in the case of eggs
with more marked polarity. Now, in artificial cultures the
eggs, after a few initial changes of water, lie upon a glass
surface undisturbed throughout their cleavage—in many cases
closely packed side by side, even when in a single layer. It is
well known how the egg of the frog is affected in its development by inversion and the consequent redistribution of yolk
in the two-cell stage ; and although the polarity of the oligolecithal Echinoderm egg is generally so little marked that it
will lie in any position in which it happens to fall, we are not
justified in assuming that, in the unnatural stillness of a culturebowl, gravity can have no effect. These conditions of immobility will also modifjr respiration. The oxygen concentration
550
H. G. NEWTH
of the water immediately adjacent to the eggs will be quickly
reduced. Oxygen reaches such eggs by diffusion downwards
from the surface of the water, possibly also by feeble convection currents ; in either case oxygenation will be greater on
that side of an egg which happens to be uppermost. Similarly,
the waste products of metabolism, escaping upwards only,
will be in greatest concentration nearest the glass surface.
Since respiration is active, there will thus be imposed on an egg
during cleavage, if not previously during maturation, a gradient
in oxygen consumption that may, or may not, coincide with
its axis. It is at least conceivable that some modification of
this original axis, or the permanent establishment of a subsidiary axis, may be the result.
If it be objected that in spite of these conditions the vast
majority of the larvae of certain species show no duplicity at
all, I can only say that, for the production of a viable double
monster in this way, optimum direction and intensity of the
modifying influence would be necessary, and only a small percentage of eggs could be expected to find this optimum by
chance orientation.
"When cleavage is completed, and the embryos escape from their
membranes, there are still external influences that may well be
teratogenetic. There are, in crowded cultures, unwonted
impacts of the blastulae upon one another and upon the walls
of their aquarium, which may cause displacement of cells or
give an unnatural stimulus to invagination ; and decantation
from bowl to bowl is probably a more violent shock to early
larvae than any they would normally sustain in the sea, involving often brusque changes of temperature, alkalinity, and even
salinity. If the view that I have put forward is correct, there
is, however, one period during which the young larva must
be peculiarly susceptable to such external influences—the period
at which the coelom appears. MacBride, in his paper on the
experimental production of twin hydrocoel in E c h i n u s ,
attributes the observed duplicity to the transference of the
Plutei to sea-water of increased salinity at a particular, critical
stage of their development. It is most significant to find that
DEVELOPMENT OP ASTROPECTEN
551
he expressly describes this stage as t h a t at w h i c h s e p a r a t i o n of t h e e n t e r o c o e l s o c c u r s . Not only is this
approximately the stage at which I have supposed a segregation
of potentialities to take place, but the agent employed—hypertonic sea-water—is one that, by causing a slight shrinkage of
the larva as a whole, might be expected to produce partial
collapse or other deformation of the thin-walled primary
coelomic rudiment, such as occurs in the case of the third-day
larva of A s t r o p e c t e n under the action of fixatives. It is
true that, in the larva described by MacBride as typical of his
cultures at the time of transference, the enterocoels are just
losing their communication with the gut; but it must be
remembered that in any culture there is considerable variation
in the rate of development of individuals, and that only a
small percentage of double larvae was obtained. In view of the
very different results obtained by Ohshima, who used the same
method, great importance must not perhaps be attached to
the use of hypertonic water. Nevertheless, in all Ohshima's
experiments the cultures—including controls—were transferred
from finger-bowls to Breffitt jars when they contained ' oneday-old larvae with pyramidal body and a pair of rudimentary
postoral arms ' (Ohshima, 24). Now this is a stage at which
the coelomic rudiment is still in connexion with the gut, and
is even nearer to what, on my assumptions, would be an
optimum moment for inhibition than that at which MacBride's
hypertonic water began to act. It will be recalled that Ohshima
obtained many more abnormalities in his cultures than MacBride.
SUMMARY.
I. (1) The normal development of A s t r o p e c t e n i r r e g u lar i s is described up to the twenty-fifth day.
(2) About a third of the larvae possessed two pore-canals,
and larval twinning was observed in two cases.
II. There is insufficient evidence for believing that normal
Echinoderm larvae possess a ' latent' bilateral symmetry.
552
H. G. NEWTH
III. The following provisional conclusions are reached regarding duplicity in Echinoderm larvae :
(1) The various kinds of duplicity form a series.
(2) They are of the same nature as those found in vertebrate
embryos, and are probably due to similar causes.
(3) They may be determined by
(o) Alteration of the polarity of the egg;
(b) Interference with processes of early development affecting gastrulation;
(c) Physiological inhibition or mechanical deformation of
the tip of the archenteron.
(4) Their ultimate facies, in the case of (c), is determined
largely by excess or defect of nutrition.
LITERATURE REFERENCES.
(Further references may be found in the papers of Gemmill (7) and
Ohshima (24).)
1. Assheton, R. (1910).—" The geometrical relation of the Nuclei in an
Invaginating Gastrula (e.g. Amphioxus) considered in connexion
with Cell Rhythm and Driesch's conception of Entelechy ", ' Arch. f.
Entw.-Mech.', vol. 29. Leipzig.
2. Child, C. M. (1915).—" Axial gradients in the early development of the
Starfish ", ' Ainer. Journ. Physiol.', vol. 37, p. 203.
3.
(1915).—' Individuality in Organisms'. Chicago: the University
of Chicago Press.
4. Delap, M. andC. (1906).—'Notes on the Plankton of Valencia Harbour.'
Fisheries, Ireland, Sci. Invest. 1905, VII.
5. Field, G. W. (1892).—"The larva of Asterias vulgaris", 'Quart.
Journ. Micr. Sci.', vol. 34. London.
6. Gommill, J. F. (1912).—" The Development of the Starfish Solaster
endeca, Forbes " , ' Trans. Zool. Soc '. London.
7.
(1914).—" The Development and Certain Points in the Adult
Structure of the Starfish, Asterias rubens, L.", ' Phil. Trans. Roy.
Soc.', B, vol. 205. London.
8.
(1915).—" Twin Gastrulae and Bipinnariae of Luidia sarsi,
Diiben and Koren ", ' Journ. Mar. Biol. Assoc.', vol. 10, no. 4.
Plymouth.
9.
(1916).—" The Larva of the Starfish Porania pulvillus (O.F.M.) ",
' Quart. Journ. Micr. Sci.', vol. 61. London.
•DEVELOPMENT OF ASTROPECTEN
553
10. Gemmill, J. E. (1916).—" Double Hydrocoel in the Development and
Metamorphosis of the Larva of Asterias rubens " , ibid. London.
11. Grave, C. (1902).—" Some Points in the Structure and Development
of Mellita testudinata " , ' Johns Hopkins Univ. Circ.', v. 21.
12.
(1911).—" Metamerism of the Echinoid Pluteus ", ibid., no. 231.
13. MacBride, E. W. (1903).—" The Development of Echinus esculentus
together with Some Points in the Development of E. miliaris and
E. aeutus ", ' Phil. Trans. Roy. Soc.', B, vol. 195. London.
14.
(1907).—"The Development of Ophiothrix fragilis " , 'Quart.
Journ. Micr. Sci.', vol. 51, pt. ii. London.
15.
(1911).—"Two Abnormal Plutei of Echinus, and the light
which they throw on the Factors in the normal development of
Echinus " , ibid., vol. 57, pt. ii. London.
16.
(1914).—' Text Book of Embryology '. London.
17.
(1918).—" The Artificial Production of Echinoderm Larvae with
Two Water-Vascular Systems, and also of Larvae Devoid of a
Water-Vascular System ", ' Proc. Roy. Soc.', B, vol. 90. London.
18.
(1921).—Note appended to Professor Ohshima's paper (23).
Ibid., vol. 92, p. 175. London.
19.
(1922).—Note appended to Professor Ohshima's paper (24).
' Quart. Journ. Micr. Sci.', vol. 66, p. 149. London.
20. Mortensen, Th. (1913).—" On the Development of some British
Echinoderms " , ' Journ. Mar. Biol. Assoc.', vol. 10, pt. i. Plymouth.
21.
(1921).—'Studies of the Development and Larval Forms of
Echinoderms '. Copenhagen.
22. Newth, H. G. (1916).—" The Early Development of Cucumaria:
Preliminary Account", ' Proc. Zool. Soc.', 1916. London.
23. Ohshima, H. (1921).—" Reversal of Asymmetry in the Plutei of
Echinus miliaris " , ' Proc. Roy. Soc.', B, vol. 92. London.
24.
(1922).—" The occurrence of Situs inversus among artificiallyreared Echinoid Larvae", ' Quart. Journ. Micr. Sci.', vol. 66.
London.
25. Rabaud, E. (1914).—" La Teratogenese", ' Encyclope'die Scientifique '. Paris.
26. Rhumbler, L. (1902).—" Zur Mechanik des Gastrulationsvorganges,
inbesondere der Invagination ", ' Arch. f. Entw.-Mech.', vol. 14,
pts. iii and iv. Leipzig.
27. Runnstrom, J. (1917-18).—" Zur Entwicklungsmechanik der Larve
von Parechinus miliaris " , ' Bergens Mus. Aarb.' Bergen.
28.
(1918).—"Analytische Studien iiber die Seeigelentwicklung " ,
IV. Mitteilung, ' Arch. f. Entw.-Mech.', vol. 43, pt. iv. Leipzig.
29.
(1918).—Ditto. V. Mitteilung, ibid.
30.
(1920).—" Entwicklungsmechanischo Studien an Henricia sanguinolenta Forbes und Solaster spec", ibid., vol. 46, pts. ii and iii.
Leipzig.
NO. 275
o O
554
H. G. NEWTH
EXPLANATION OF PLATES 40 AND 41.
LETTERING EMPLOYED.
u.d., anterodorsal process ; an., anus ; or., archenteron ; c.b., circumoral ciliated band; Co./., circumoral field; ex., evaginating cells;
f.a., frontal area; f.m., fertilization membrane ; H., hydropore ; Hr.,
right hydropore ; IN., intestine; L.G., left enterocoel; m.l., median
loop of the postoral band; m.p.p., median preoral process ; Oes., oesophagus ; P.C., primary coelomic ve3icle ; P.O., postoral ciliated band ;
S.G., right enterocoel; S., stomach ; St., stomodaeum ; su., cavity of
sulcus.
PLATE 40.
All drawings were made with the camera lucida.
Fig. 1.—Blastula of A s t r o p e c t e n , fifteen hours old. Section showing
evagination in progress. The cells in the blastocoel are parts of the blastulawall. Magnification 330—in figures. Drawn under Zeiss 3 mm. apochr.,
Leitz comp. oe. 6.
Fig. 2.—Longitudinal section of early gastrula, showing both invagination and evagination in progress. Magnification, &c, as in fig. 1.
Figs. 3-6.—Third-day larvae all taken from the same preserved sample
and illustrating early stages in the development of Bipinnaria. Drawn
from stained and cleared specimens under Leitz 8 mm. apochr., comp. oc. 6.
Magnification 136—in figures. Ectoderm shown in optical section (black),
its thickness accurately indicated.
Fig. 3.—Completed gastrula. Primary vesicle formed.
Fig. 4.—Young post-gastrula stage, seen from the left side. Wings
of the primary vesicle have grown out right and left, the left being the
larger. The endodermal oesophagus touches the stomodaeum.
Fig. 5.—Post-gastrula stage slightly later than that shown in fig. 4 and
seen from the dorsal surface. Oesophagus constricted off from stomach ;
hydropore just appearing and enterocoels losing connexion with gut;
ciliated band faintly indicated above stomodaeum.
Fig. 6.—Abnormal post-gastrula at the same stage as the normal larva
shown in fig. 4. Ventral aspect. For description see text, p. 527.
PLATE 41.
Figs. 7-11 are all camera lucida drawings made from animals still in
70 per cent, alcohol. Magnification 100—in figures. Leitz obj. 3, oc. 15.
Fig. 7.—Bipinnaria nearly six days old ; ventral aspect.
Fig. 8.—Bipinnaria twenty-four days old ; ventral aspect.
Fig. 9.—Bipinnaria twenty-four days old ; ventro-lateral aspect. This
is not the same individual as that shown in fig. 8, though almost identical
in size and shape.
Fig. 10.—Abnormal Bipinnaria, eight days old ; right dorso-lateral
aspect. For description see text, p. 527.
Fig. 11.—The larva shown in fig. 10 ; ventral aspect.
Quart. Journ. Micr. Sci.
FIG. 5
Newtk, del.
Vol. 69, N. S., PI 40
FIG. 6
Quart. Journ. Mkr. Sci
Vol. 69, N. S., PL 41
FIG. II