IX. DEVELOPMENT OF THE TELEOSTEAN EGG
409
separately, only the explants of the layer stretch; those of the blastomeres show neither regularization of shape, nor tendency to stretch
(Devillers et al., 1957b). The increase in surface of whole explants
(enveloping layer + deep blastomeres) is clearly inferior to that of a
normal germ spreading over the yolk. One can also observe that extension and organization are nearer to normal in the head than in the trunk
(see also Oppenheimer, 1936) (Fig. 8(6)). Development of organs plays ro
part, since differentiation goes quite far in some cases. One must then
consider that the full realization of spreading requires a solid substrate
(see also Oppenheimer, 1938) and that traction by the perivitelline
pellicle no doubt plays a part.
The nature of the epibolic capacity of the enveloping layer is connected with the more general problem of cellular motility (see also
Holtfreter, 1943b, c, 1946, 1948); the individual movements of the cells
are co-ordinated to form a general movement by the strong intercellular
bonds.
We already know of the autonomous extension properties of the
cortex and the enveloping layer in the pregastrular germ (see Section
III, C, 2, e); it is likely that the epibolic capacity is nothing but the
natural manifestation of these latent properties. During epiboly, the
enveloping layer still has the capacity to heal a wound, that is to say it
can still carry out movement in the opposite direction to its general
movement.
Holtfreter has shown (1948) that the cellular wall provides the motive
force of elongation, and, on the other hand, we know that the cortex
possesses the faculty of expansion to a greater degree than the other
blastomere walls; so we think that the intrinsic force of epiboly resides
essentially in the peripheral wall (facing the perivitelline liquid) of the
cells of the enveloping layer.
Is cellularization absolutely necessary for the occurrence of epiboly?
In Salmo it is: non-cleaved eggs survive a long time without showing the
beginnings of movements observed by Holtfreter (1943a) in the undivided eggs of Amphibia. However Trifonova (1934) speaks of parthenogenetic eggs (non cleaved) of Acerina, Perca, Abramis, that show
the closing of the blastopore and the organization of an embryo. According to Holtfreter's idea, cleavage is not absolutely necessary for the
realization of morphogenetic movements; it only makes them easier.
The case of the Teleosts shows that it is sometimes quite unnecessary.
It must be mentioned that, by the Unna-Brachet reaction, the enveloping layer shows a greater richness in ribonucleic granules than the
other blastomeres (Devillers, 1951a). This may be a characteristic of
'anläge' endowed with an important kinematic activity (Dalcq et al.,
1949).
409
separately, only the explants of the layer stretch; those of the blastomeres show neither regularization of shape, nor tendency to stretch
(Devillers et al., 1957b). The increase in surface of whole explants
(enveloping layer + deep blastomeres) is clearly inferior to that of a
normal germ spreading over the yolk. One can also observe that extension and organization are nearer to normal in the head than in the trunk
(see also Oppenheimer, 1936) (Fig. 8(6)). Development of organs plays ro
part, since differentiation goes quite far in some cases. One must then
consider that the full realization of spreading requires a solid substrate
(see also Oppenheimer, 1938) and that traction by the perivitelline
pellicle no doubt plays a part.
The nature of the epibolic capacity of the enveloping layer is connected with the more general problem of cellular motility (see also
Holtfreter, 1943b, c, 1946, 1948); the individual movements of the cells
are co-ordinated to form a general movement by the strong intercellular
bonds.
We already know of the autonomous extension properties of the
cortex and the enveloping layer in the pregastrular germ (see Section
III, C, 2, e); it is likely that the epibolic capacity is nothing but the
natural manifestation of these latent properties. During epiboly, the
enveloping layer still has the capacity to heal a wound, that is to say it
can still carry out movement in the opposite direction to its general
movement.
Holtfreter has shown (1948) that the cellular wall provides the motive
force of elongation, and, on the other hand, we know that the cortex
possesses the faculty of expansion to a greater degree than the other
blastomere walls; so we think that the intrinsic force of epiboly resides
essentially in the peripheral wall (facing the perivitelline liquid) of the
cells of the enveloping layer.
Is cellularization absolutely necessary for the occurrence of epiboly?
In Salmo it is: non-cleaved eggs survive a long time without showing the
beginnings of movements observed by Holtfreter (1943a) in the undivided eggs of Amphibia. However Trifonova (1934) speaks of parthenogenetic eggs (non cleaved) of Acerina, Perca, Abramis, that show
the closing of the blastopore and the organization of an embryo. According to Holtfreter's idea, cleavage is not absolutely necessary for the
realization of morphogenetic movements; it only makes them easier.
The case of the Teleosts shows that it is sometimes quite unnecessary.
It must be mentioned that, by the Unna-Brachet reaction, the enveloping layer shows a greater richness in ribonucleic granules than the
other blastomeres (Devillers, 1951a). This may be a characteristic of
'anläge' endowed with an important kinematic activity (Dalcq et al.,
1949).