/ . Embryol. exp. Morph. Vol. 20, 3, pp. 247-60, November 1968
With 2 plates
Printed in Great Britain
247
Primordial germ cells in normal and
transected duck blastoderms
By TERESA ROGULSKA 1
From the Department of Embryology, University of Warsaw
Suggestive evidence for the extragonadal origin of germ cells in birds was
first presented by Swift (1914), who described primordial germ cells in the chick
embryo at as early a stage as the primitive streak. According to Swift, primordial
germ cells are originally located extra-embryonically in the anterior part of the
blastoderm and occupy a crescent-shaped region ('germinal crescent') on the
boundary between area opaca and area pellucida. Swift also found that primordial
germ cells later enter into the blood vessels, circulate together with the blood
throughout the whole blastoderm and finally penetrate into the genital ridges,
where they become definitive germ cells. Swift's views have been confirmed in
numerous descriptive and experimental investigations. Among the latter, the
publications of Willier (1937), Simon (1960) and Dubois (1964a,b, 1965a, 6,
1966) merit special attention. Dubois finally proved that the genital ridges
exert a strong chemotactic influence on the primordial germ cells.
Duck embryos have not as yet been so intensively investigated in this respect
as those of chicks. The recently published reports by Fargeix (1966, \961a,b,c)
and Fargeix & Theilleux (1966) suggest that the history of primordial germ cells
in the duck is similar to that in the chick.
Although the history of primordial germ cells (PCGs) in birds has been
thoroughly investigated, the cause of their differentiation at a given time and
in a given territory is still unknown. One of the possible ways of attacking this
problem is to disturb the original organization of the blastoderm by transection,
which results in the formation of twin embryos. Studies on the transection in ovo
of bird blastoderms have been made for several years by Lutz and his co-workers
(Lutz, 1949, 1953, 1955, 1964, 1965; Fargeix, 1964a,b, 1965, I961a,b,c) and
have shown that in the duck, as well as in the quail, goose and turkey,
the embryo can form from any sufficiently large fragment of the blastoderm.
The method used by these authors (Wolff & Lutz, 1947; Lutz, 1949) makes it
possible to obtain twin embryos, which may survive as long as 10 days of
incubation.
Germ cells in duck twin embryos, obtained as a result of transection of the
1
Author's address: Department of Embryology, University of Warsaw, Warsaw 64,
Poland.
248
T. ROGULSKA
blastoderm, have been investigated mainly in embryos at an advanced stage of
development (Lutz, 1949; Lepori, 1961; Pala, 1963; Fargeix, 1964a,b). It was
found that both twin embryos contained germ cells, which is quite natural in
view of their common circulatory system. The original localization of these
cells before migration had not been examined, however, and forms the chief
object of this study. During its preparation for publication two papers by
Fargeix (1967 a, b) appeared, in which the author particularly describes the
localization of primordial germ cells in single embryos obtained after elimination
of one half of the blastoderm.
MATERIALS AND METHODS
Duck eggs of the Pekin strain were used for the experiments. Unincubated
blastoderms were transected by the method of Wolff & Lutz (1947) and Lutz
(1949), either perpendicular or parallel to the long axis of the shell (Rogulska,
1968). The eggs were incubated for 48-98 h at 37-8 °C. Embryos developing
after operation were always smaller than the control embryos, retarded in development and often abnormal. The operated and control blastoderms were fixed
in Bouin or Gendre fluid, embedded in paraffin, sectioned at 6 [i and stained
with Ehrlich's haematoxylin and eosin. Some of the blastoderms were photographed before embedding in paraffin wax, having previously been stained in
toto with Ehrlich's haematoxylin and cleared in benzene for this purpose. A
total of thirty-one normal and forty-two transected blastoderms were examined.
Primordial germ cells were identified by virtue of their dimensions, spherical
shape and the presence of yolk granules or vacuoles. Consecutive sections containing primordial germ cells were drawn by means of a drawing apparatus and
the position of cells marked on the drawing. The total number of sections into
which the blastoderm was cut, as well as the position of PGCs within each
section having been established, it was then possible to determine the approximate position of PGCs within the blastoderm by entering the position of each
cell on a large photograph of the whole blastoderm. The data thus assembled
were then converted into diagrams of the type shown in Text-fig. 1.
RESULTS
1. Primordial germ cells in normal duck blastoderms
The control material consisted of thirty-one blastoderms in stages varying
from head process to thirty-five somites (Table 1).
Until active circulation begins, PGCs are scattered in the anterior part of
the blastoderm, occupying the germinal crescent, as Fargeix (1966) has also
recently reported. Even before circulation begins, however, some changes are
observed in the localization of PGCs caused more by the change in the shape
of the territory occupied by them than by active migration. In the pre-somite
stages the PGCs are mainly located in the anterior part of the area pellucida,
Blastoderm germ cells
249
which is relatively wide in front of the embryo (Plate 1 and Text-fig. 1, A) and
is not yet completely invaded by mesoderm. In stages with more than six somites
the area pellucida in front of the embryo is far narrower, and PCGs are situated
mainly laterally on both sides of the embryo's head. Fargeix (1966) observed
that the posterior ends of the germinal crescent gradually become wider and he
related this with the growth and lateral spreading of the mesoderm, taking some
of the PGCs with it. It is possible that both factors—change in shape of area
pellucida and growth of the mesoderm—jointly cause a change in the shape
of the germinal crescent.
Table 1. Numbers of primordial germ cells in control blastoderms
Stage
(no. of somites)
No. of
blastoderms
No. of
PGCs
0*
3
1
3
1
1
4
6
7
8
9
10
16
18
20
21
22
23
24
25
27
2
2
2
2
1
1
1
1
2
1
1
1
1
1
1
6
36, 60, 88
106
42
26, 53
40,79
74,159
84, 140
30-35
Total
31
Mean no.
of PGCs
i
59
85
70
51
59
)
10
62,65
13
38
45
9
32
70
38
J
138
104,185, 57
159,173,328
—
81
* Head process stage.
Even in the early stages, before circulation starts, PGCs could sometimes be
found in the posterior part of the blastoderm. They usually lie freely above the
mesoderm (Plate 2, fig. L), often very far from the germinal crescent, and never
occur in any great numbers. The fact that their occurrence is only sporadic
suggests that they may be cells which accidentally moved from the germinal
crescent towards the posterior part of the blastoderm. Fargeix (1966), however,
considers that in some duck blastoderms a considerable proportion of the
PGCs is situated in the posterior part of the blastoderm, and that the cells in
question originate in this particular territory.
In the 9- to 10-somite stages almost all PGCs are already in the blood vessels.
250
T. ROGULSKA
Text-fig. 1. Representative blastoderms, showing localization of PCGs (cf. Plate 1).
Stippled areas indicate distribution of PGCs (the dots do not represent individual
cells).
J. Embryol. exp. Morph., Vol. 20, Part 3
PLATE 1
*,;••!. mto®mte.<>, E
T. ROGULSKA
Blastoderm germ cells
251
They were not observed to penetrate actively into the vessels; it seems more
likely that they are passively enclosed while the vessels form. After active circulation has started at the stage of about 16 somites, PGCs circulate in the
blood throughout the whole blastoderm, and from the 27-somite stage onwards
they begin to appear in the genital ridges. Single PGCs may 'get lost' during
migration and reach non-typical sites, e.g. the head mesenchyme (Plate 2,fig.K).
This phenomenon has been described by many investigators in chick embryos
(see Meyer, 1964). By the 35-somite stage penetration of PGCs into the ridges
is nearly completed and only single PGCs may still remain in the blood vessels.
According to van Limborgh (1958, 1961) the distribution of PGCs between the
genital ridges of the duck embryo is symmetrical up to the stage of 36 somites.
At the 38-somite stage the primordium of the left gonad already contains 60%
of the total number of PGCs. In the embryos which I examined this asymmetry
must have begun somewhat earlier, since in all six 31- to 35-somite embryos the
left ridge already contained slightly more PGCs than its counterpart (34/23,
55/14, 88/85, 105/54, 109/73, 179/149).
EXPLANATION OF PLATES
PLATE 1
Fig. A. Control embryo. One somite, x 12
Fig. B. No. 7 (group I). Eight and ten somites, x 12
Fig. C. No. 21 (group II). Three and eight somites, x 12
Fig. D. No. 11 (group I). Thirteen and fourteen somites, x 2-5
Fig. E. No. 15 (group I). Fourteen and eighteen somites, x 2-5
Fig. F. No. 26 (group III). Three and six somites, x 2-5
Fig. G. No. 33 (group III). Twenty, twenty-two, twenty-six somites, x 1-5
Fig. H. No. 40 (group IV). Eight and twelve somites, x 2-5
PLATE 2
Fig. I. Primordial germ cell in entoderm of area pellucida of normal blastoderm (head
process stage), x 1200.
Fig. J. Primordial germ cells above entoderm of area pellucida of normal blastoderm
(8-somite stage). x800.
Fig. K. Primordial germ cells (arrows) in head mesenchyme of normal embryo (20-somite
stage). x400.
Fig. L. Primordial germ cell above lateral mesoderm of normal embryo (7-somite stage).
x400.
Fig. M. Primordial germ cell in blood vessel. A large vacuole can be seen. Transected
blastoderm no. 38 (group IV), six and eleven somites, x 1200.
Fig. N. Primordial germ cell in karyokinesis (arrow). Transected blastoderm no. 38 (group
IV), six and eleven somites, x 1200.
Fig. O. Group of primordial germ cells above mesoderm (arrow). Transected blastoderm
no. 38 (group IV), six and eleven somites, x 300.
Fig. P. Numerous primordial germ cells in blood vessels. Transected blastoderm no. 3
(group I), degenerate somites, x 400.
252
T. ROGULSKA
Considerable individual variations in the number of PGCs were observed
among the blastoderms in all stages examined. The increase in the number of
PGCs during the period under investigation is slight (Table 1). The decrease in
the mean number of PGCs during the 16- to 24-somite stages is more apparent
than real, and is possibly due to failure to observe all the PGCs distributed
through the blood vessels of the whole blastoderm during this period. It would
appear that the number of PGCs at the moment of their immigration into the
genital ridges is determined to a greater extent by the number of cells originally
formed than by mitotic divisions of PGCs during their migration through the
blood vessels.
The characteristics of the PGCs change slightly during migration. During the
period of their formation in the germinal crescent they contain a relatively
large amount of yolk (Plate 2, fig I, J), which is gradually used up during
migration. PGCs circulating in the blood vessels (Plate 2, fig. M) and especially
those which have settled in the genital ridges contain very little yolk but possess
large vacuoles in the cytoplasm instead.
2. Primordial germ cells in transected blastoderms
Forty-two blastoderms were classified into four groups according to the
arrangement of the embryos (Rogulska, 1968). Examination was made of
nineteen blastoderms with embryos lying parallel or almost parallel to each
other, five blastoderms with embryos head to head, nine with embryos arranged
head to tail (blastoderms with posterior duplication of the anterior embryo,
such as shown in Plate 1, fig. G, were also included in this group), and nine
blastoderms with embryos arranged at various angles.
The characteristic features of PGCs in transected blastoderms and their
history are the same as in the control blastoderms, although the territory they
originally occupy is not always similar in shape to the germinal crescent. Analysis
of the distribution of PGCs in transected blastoderms often meets with difficulties. Twin embryos often differ as to size and degree of development. Not
infrequently they are markedly retarded in development and their development
does not always proceed normally. Disturbances in structure and function of
the circulatory system have a decisive effect on the distribution of PGCs in the
blastoderm, and absence of circulation, leading in the end to the death of the
embryos, results in the PGCs remaining in their original position.
(a) Number of primordial germ cells
The number of PGCs in some of the transected blastoderms was very large
as compared with control material. Considerable individual fluctuations also
occurred (Table 2), but the mean total number of PGCs for all the transected
blastoderms is 168-2, as compared with 81-2 for the control blastoderms.
Different mean numbers of PGCs were found in different groups, with group I
exhibiting the highest mean number. It is, however, difficult to evaluate the
Blastoderm germ cells
253
Table 2. Numbers of primordial germ cells in transected blastoderms
No. of
blastoderm
Hours
of inc.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30*
31*
32*
33*
34
35
36
37
38
39
40
51
64
72
56
53
66
66
66
67
65
66
73
72
86
71
66
93
96
98
48
55
74
66
73
57
57
70
73
97
86
69
93
95
48
56
65
72
65
74
66
41
96
42
96
No. of
somites
{x = degenerate
somites)
Type of
arrangement of
twin embryos
(group)
X, X
X, X
X, X
4,9
6,9
9,9
8,10
6,12
6,12
I
(parallel or
nearly)
X, X
(
I
I
0,5
3,8
\
11
>
(head to
head)
J
13,14
14,14
14,15
13,18
14,18
17,19
26,27
26,30
35,36
/
4,19
16,16
11,21
0,0
3,6
5,5
III
(head to
tail)
4,11
x,29
3, 6,10
x, 8, 16
5,10,19
V 20,22,26
/
0,4
3,4
2,8
IV
(at various
angles)
f
V
3,12
6,11
4,14
8,12
20,27
30,35
No. of
PGCs
326
143
852
276
180
158
88
52
84
307
165
112
86
428
382
58
95
43
154
182
231
85
40
56
214
Mean no.
ofPCGs
\
208
1
>|
•
J
\
35
8
189
27
88
78
83
336
14
117
>
\
127
153
72
292
172
80
86
444
119
160
>
* Posterior duplication of the anterior embryo
17
TEEM 23
254
T. ROGULSKA
significance of these differences, since the sample was small and great differences
occur between blastoderms. The general increase in mean number of PGCs
appears, however, to be significant.
The number of PGCs in operated blastoderms does not depend on the general
condition of the blastoderm. Blastoderms very retarded in development, with
abnormal embryos and poor circulation, did not contain fewer PGCs than those
which had developed more normally. Not infrequently the absolute age of such
blastoderms (in hours of incubation) was very high in comparison with their
stage of development as indicated by the number of somites. If PGCs had
undergone intensive mitotic divisions, then their number should systematically
increase with the increasing age of the embryo. As Table 3 shows, such an
increase does not occur. In addition, examination of the mitotic index of PGCs
in several normal and transected blastoderms did not point to significant
differences between them.
Table 3. Numbers of primordial germ cells in transected blastoderms according
to the time of incubation
Hours of
incubation
No. of
blastoderms
48-58
9
58-68
12
68-78
11
78-88
88-98
2
Total
8
No. of PGCs
14, 35,127,180,182,
214, 231, 276
40, 52, 58, 80, 84, 88,
143, 153,158,165,
292, 307
8, 56, 72, 78, 85, 86,
112,172,189,382,852
88,428
27, 43, 83, 86, 95,154,
336, 444
42
—
Mean no.
ofPCGs
175
135
190
258
158
169
The number of PGCs therefore mainly depends on how many of them were
produced initially. In transected blastoderms this number is doubled on the
average, because the territory where they are formed is much larger or because
two territories are present.
No connexion could be observed between the degree of development or the
position of twins and the number of PGCs found around them at the time of
primary localization.
(b) Primary localization of primordial germ cells
The basic difference between transected and control blastoderms consists in
the different primary localization of PGCs. It is self-evident that this can only
be observed in those stages in which the original localization of PGCs has not
yet been affected by the activity of the circulatory system.
Blastoderm germ cells
255
Primary localization of PGCs was observed in twelve blastoderms belonging
to group I. If the twin embryos lie in a common area pellucida, the highest
number of PGCs is located in front of the embryos, and a smaller number
between them. If two areae pellucidae have formed, then relatively more PGCs
occur between the embryos; with parallel or nearly parallel embryos the PGCs
are distributed mainly along the line of transection of the blastoderm (Plate 1
and Text-fig. 1, B, D, E). This phenomenon does not occur with other
arrangements of the embryos.
In four blastoderms in group II (embryos arranged head to head) the PGCs
are encountered chiefly in the boundary region between area pellucida and area
opaca, or in the area pellucida at the level of the anterior parts of the embryos
(Plate 1 and Text-fig. 1, C). In group II the line of transection and the localization
of the PGCs are not clearly associated.
In group III (embryos arranged head to tail) six blastoderms were found
showing primary localization of PGCs. The line of transection of the blastoderm
did not affect the position of the PGCs (Plate 1 and Text-fig. 1, F, G). No connexion was observed between the number or distribution of PGCs and the
position of either embryo. An embryo formed from the posterior part of the
blastoderm is usually more advanced, but is not always found to have a higher
number of PGCs in its vicinity.
The mixed group IV contains embryos arranged at various angles, usually at
right angles. The primary localization of the PGCs was observed in seven
blastoderms. In four of them a distinct connexion could be observed between
the line of transection and the position of the PGCs: they occur particularly
in the 'central' part, along the line of transection (Plate 1 and Text-fig. 1, H).
(c) Comparison of behaviour and fate of primordial germ cells in transected and
normal blastoderms
Although in general outline the fate of PGCs in transected blastoderms is
the same as in controls, certain deviations from the normal pattern were
observed. The first PGCs in blood vessels are encountered as early as the 3-somite
stage, earlier than in control blastoderms. This suggests that the formation of
somites was inhibited, while the formation of blood vessels was not. The penetration of PGCs into the blood vessels normally ends at the 6- to 10-somite stage;
PGCs are found outside the blood vessels as late as the 10-somite stage in
transected blastoderms. No connexion was observed in transected blastoderms
between the width of the area pellucida in front of the embryo and the occurrence
of PGCs in it; blastoderms were often encountered which had a large area
pellucida in front of the embryos with only a small number of PCGs. In normal
development the 16-somite stage marks the beginning of active circulation.
From this time PGCs begin to move away from the germinal crescent and to
circulate with the blood. In transected blastoderms circulation does not always
begin at the 16-somite stage; this may take place at stages with several somites
17-2
256
T. ROGULSKA
less or more. In addition, in some cases circulation was so poor that PGCs
remained in the vicinity of the former germinal crescent.
It has been assumed that it is not until circulation has been established that
PGCs can be found in intraembryonic vessels (Meyer, 1964). In transected
blastoderms, however, PGCs were repeatedly found in the heart or dorsal
aortae of embryos at such an early stage that it would be difficult to assert that
they could have reached there in the normal way (Plate 1 and Text-fig. 1, H).
This could probably have taken place passively, by the enclosing of some PGCs
in the aortae or heart as they formed. From the 31-somite stage onwards normally
almost all PGCs should have settled in the genital ridges. In transected blastoderms the invasion of the genital ridges by PGCs may take place at slightly
later stages. In general, however, PGCs in transected blastoderms pass through
the same phases of migration as in control blastoderms, although the consecutive
phases may be reached at different stages of development on account of the
retarded development of twin embryos and the asynchrony of developmental
processes. The only two significant features by which transected blastoderms
differ from normal ones appear to be the general increase in the number of
PGCs and the effect of the line of transection, which often takes over the
functions carried out in normal development by the germinal crescent only.
DISCUSSION
As shown by the present experiments and those of Fargeix (1967a,b), any
half of the unincubated blastoderm can give rise not only to an embryo, but
also to primordial germ cells. These results become more understandable if it
is assumed that in the unincubated blastoderm PGCs have not yet been individualized, and that the embryo must reach a certain stage before their formation
can begin. This suggestion is not in agreement with observations by Simon
(1960), who states that in the unincubated chick blastoderm these cells are
already present, and scattered over the whole blastoderm. New data on this
subject were recently presented by Dubois (1967 a, b) on the basis of in vitro
experiments. According to his hypothesis the primordial germ cells originate in
the lower posterior part of the unincubated chick blastoderm and are then
transported together with the lower layer towards the germinal crescent by the
pregastrulation movements. One of the pieces of evidence for this interesting
hypothesis is the fact that after culturing anterior and posterior parts of unincubated blastoderms separately during 48 h the posterior part is found to contain
on average more PGCs than the anterior one. However, when a similar experiment was performed in ovo, Fargeix (1967 b) came to the conclusion that the
posterior part contains on average fewer PGCs. It will be remembered that the
duck blastoderm is less advanced at egg-laying than that of the chick; consequently, according to Dubois' hypothesis more PGCs would be expected in the
posterior part. As far as the present experiments are concerned no significant
Blastoderm germ cells
257
differences between anterior and posterior parts have been observed, although
it must be stressed that the number of embryos showing head-to-head and
head-to-tail alignment was rather small.
Primordial germ cells arise normally from that part of the boundary region
between the area opaca and the area pellucida which is farthest away from the
growth centre. The germinal crescent is the region where the completion of the
lower layer and the appearance of the mesoderm take place last. In transected
blastoderms, however, the primary localization of PGCs cannot always be
explained solely by the formation of a germinal crescent in front of each embryo.
In blastoderms showing parallel or nearly parallel arrangement of the embryos
an association can be observed between the position of the PGCs and the former
line of transection of the blastoderm. The PGCs are located mainly along this
line and can be encountered even in the vicinity of the posterior parts of the
embryos (Plate 1 and Text-fig. 1, D). Their concentration along the former line
of transection, between the embryos, cannot be explained only by an overlapping
of the two germinal crescents, since PGCs are often situated far backwards and
rarely occur at all on the 'external' sides. Thus the line of transection may
apparently sometimes play a role similar to that of the germinal crescent in
normal development. A similar phenomenon was observed by Fargeix (1967 a)
even when one of the halves of the blastoderm was eliminated after transection;
the developing embryo exhibited a distinct asymmetry of the crescent caused
by the more numerous occurrence of PGCs on the transected side. One of the
possible explanations of this phenomenon is that disturbances caused by the cut
in some way create favourable conditions for the formation of PGCs. It must
be emphasized, however, that the phenomenon described occurs only with this
particular alignment of the embryos. When they are aligned head to head or
head to tail no connexion is observed between the primary localization of the
PGCs and the line of transection. This was also noted by Fargeix (19676) when
embryos deriving from one half of a blastoderm were examined.
The numbers of PGCs in both control and transected blastoderms exhibit
considerable individual variations, but the transected blastoderms contained on
an average twice as many PGCs as the normal blastoderms. In my control
material the increase in the number of PGCs before their settlement in the
genital ridges was very slight. Fargeix & Theilleux (1966), on the other hand,
when examining the number of PGCs in normal duck blastoderms in stages
ranging from the primitive streak to fifteen somites, observed a distinct increase
in the average number of PGCs, from twenty for the primitive streak stage to
as many as 350-400 for stages with over ten somites. Fargeix & Theilleux
consider that the formation of new PGCs in the germinal crescent lasts only up
to the 4-somite stage, and that the further increase in their number is caused by
mitotic divisions of pre-existing cells. They explain the relatively low mitotic
index of PGCs by difficulties in identifying the dividing cells. Fargeix (1967o),
on the basis of his experiments involving simple transection of the blastoderm
258
T. ROGULSKA
or destruction of one of the halves, states that the embryos formed from one
half of the blastoderm contain as many PGCs as normal ones. Therefore, after
transection the whole blastoderm would have contained twice as many PGCs,
which is in agreement with my results.
It seems to me that the number of primordial germ cells in the blastoderm,
whether normal or transected, depends to a greater extent on the number of
PGCs originally formed than on mitotic divisions of pre-existing PGCs.
Transection itself may in certain cases stimulate formation of PGCs and thus
evoke an increase in their number from the very beginning of development.
SUMMARY
1. In early stages of development of the duck primordial germ cells (PGCs)
initially occupy a crescent-shaped area, the germinal crescent. The characteristic
features of PGCs and their fate from the time of formation up to the time of
settlement in the genital ridges are the same as in the chick embryo. The number
of PGCs exhibits considerable individual variations and does not markedly
increase until after the PGCs settle in the ridges.
2. The primary localization of PGCs in transected blastoderms may be different
from that in normal blastoderms and is affected by the arrangement of the twin
embryos. With parallel or nearly parallel twins the majority of PGCs are located
between the embryos, in the area through which the line of transection ran.
This connexion between the position of the PGCs and the line of transection
is not found when the embryos are arranged head to head or head to tail.
3. The number of PGCs in transected blastoderms is on an average twice as
high as the number of PGCs in normal blastoderms.
4. Each half of an unincubated blastoderm is equally capable of forming an
embryo and of forming PGCs.
5. Neither the characteristic features of PGCs in transected blastoderms nor
the course taken by their migration deviate from those seen in normal
development.
RESUME
Les cellules germinates primordiales chez les blastodermes normaux ou
fissures du Canard
1. Pendant les stades precoces du developpement du Canard, les cellules
germinales primordiales (CGP) occupent initialement une region en forme de
croissant, le croissant germinal. Les caracteristiques des CGP et leur destinee
depuis leur formation jusqu'a ce qu'elles s'etablissent dans les cretes genitales
sont les memes que chez l'embryon de Poulet. Le nombre de CGP par individu
est tres variable, et ne montre pas d'augmentation importante avant l'etablissement des CGP dans les cretes.
2. La localisation primaire des CGP dans les blastodermes fissures peut
Blastoderm germ cells
259
etre differente de celle chez les blastodermes normaux selon la disposition relative
des embryons jumeaux. Les embryons places paralleles ou presque paralleles,
la plupart des CGP se trouve entre les embryons dans la region ou se fit la ligne
de fissuration. Cette relation entre les CGP et la ligne de ne se reproduit pas
lorsque les embryons sont places tete a tete ou tete a queue.
3. Le nombre de CGP dans les blastodermes fissures est en moyenne deux
fois les nombre de CGP chez le blastodermes normaux.
4. Chaque moitie d'un blastoderme non-incube est capable egalement de la
formation d'un embryon et de la formation des CGP.
5. Ni les caracteristiques des CGP dans les blastodermes fissures ni le cours
de leur migration ne differe de ce que Ton voit pendant le developpement
normal.
I wish to express my sincere thanks to Dr Andrzej K. Tarkowski for his advice and
encouragement throughout every stage of this work. I am also grateful to Professor
P. D. Nieuwkoop and Dr J. Faber (Hubrecht Laboratory, Utrecht) for helpful comments
and assistance in the preparation of the manuscript.
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{Manuscript received 6 February 1968)