IX. DEVELOPMENT OF THE TELEOSTEAN EGG
421
that the enveloping layer takes part in normal development through its
mechanical properties or through its impermeability properties (i.e.
by stopping the diffusion of the substances that play a part in cellular
mobilization) 1
The syncytial surface is adhesive and it is the fixed base on which the
cells move during invagination (see also Oppenheimer, 1935). It certainly plays an important part, but it cannot really be thought that it
directs the cellular movements since, in explants with an enveloping
layer but deprived of this support, the arrangement of the marginal
material is carried out normally under conditions which recall those of
the Amphibian egg, but which have not been followed with precision.
Treatment of eggs by. KCN, NaN 3 , 2-4DNP or rearing them in a
medium lacking 0 2 enables one to dissociate the movements of epiboly
and invagination (Devillers et ah, 1953b, 1957a).
V. Conclusion
In his general review published in 1940, Pasteéis underlines the
similarity of the plans of presumptive regions in Amphibia and in
Teleosts. There is a plan, characteristic of Anamniota, from which that
of Amniota shows important differences.
In spite of dissimilarities of aspect and of behaviour, the eggs of
Amphibia and those of Teleosts are 'homologous'. By different paths,
premorphogenesis leads in both cases to an identical distribution of
presumptive regions. If we adopt, as a practical hypothesis, the egg of the
Amphibia as the primitive (ancestral) type, we can suppose that a
hypertrophy of the yolk, localized in the sub-blastoporal zone, has little
by little pushed back the cytoplasm towards one pole; morphological
stages of this evolution are provided for us by the egg of Acipenser
(Detlaf and Ginsburg, 1954), still very near to that of Amphibia, and by
the egg of Amia (Whitman and Eycleshymer, 1897) already much more
transformed: the greater part of its mass starts to become inert and
foreshadows the vitelline sac, while the active portion has already the
aspect of a blastoderm. It is not only the increase in the vitelline charge
that leads to the telolecithal egg, but also a physico-chemical peculiarity
of the cytoplasm-yolk mixture: the yolk droplets merge and the two
components become entirely separate.
This new condition in the relations between the two components
deeply modifies certain processes of premorphogenesis, without altering
however the 'pattern' of the primitive egg. But a real novelty appears:
the vitelline syncytium (of which there is perhaps an outline in Amia
2
.)
which is the physical and physiological intermediary between cytoplasm
and yolk.
421
that the enveloping layer takes part in normal development through its
mechanical properties or through its impermeability properties (i.e.
by stopping the diffusion of the substances that play a part in cellular
mobilization) 1
The syncytial surface is adhesive and it is the fixed base on which the
cells move during invagination (see also Oppenheimer, 1935). It certainly plays an important part, but it cannot really be thought that it
directs the cellular movements since, in explants with an enveloping
layer but deprived of this support, the arrangement of the marginal
material is carried out normally under conditions which recall those of
the Amphibian egg, but which have not been followed with precision.
Treatment of eggs by. KCN, NaN 3 , 2-4DNP or rearing them in a
medium lacking 0 2 enables one to dissociate the movements of epiboly
and invagination (Devillers et ah, 1953b, 1957a).
V. Conclusion
In his general review published in 1940, Pasteéis underlines the
similarity of the plans of presumptive regions in Amphibia and in
Teleosts. There is a plan, characteristic of Anamniota, from which that
of Amniota shows important differences.
In spite of dissimilarities of aspect and of behaviour, the eggs of
Amphibia and those of Teleosts are 'homologous'. By different paths,
premorphogenesis leads in both cases to an identical distribution of
presumptive regions. If we adopt, as a practical hypothesis, the egg of the
Amphibia as the primitive (ancestral) type, we can suppose that a
hypertrophy of the yolk, localized in the sub-blastoporal zone, has little
by little pushed back the cytoplasm towards one pole; morphological
stages of this evolution are provided for us by the egg of Acipenser
(Detlaf and Ginsburg, 1954), still very near to that of Amphibia, and by
the egg of Amia (Whitman and Eycleshymer, 1897) already much more
transformed: the greater part of its mass starts to become inert and
foreshadows the vitelline sac, while the active portion has already the
aspect of a blastoderm. It is not only the increase in the vitelline charge
that leads to the telolecithal egg, but also a physico-chemical peculiarity
of the cytoplasm-yolk mixture: the yolk droplets merge and the two
components become entirely separate.
This new condition in the relations between the two components
deeply modifies certain processes of premorphogenesis, without altering
however the 'pattern' of the primitive egg. But a real novelty appears:
the vitelline syncytium (of which there is perhaps an outline in Amia
2
.)
which is the physical and physiological intermediary between cytoplasm
and yolk.