IX. DEVELOPMENT OF THE TELEOSTEAN EGG
407
achieved and the vitelline syncytium is formed; polarity and symmetry
are fixed and the marginal zone is determined.
Since the egg is isolated from the medium by its impermeable cortex,
intrinsic factors must start the morphogenetic movements characteristic
of the vertebrate egg (Pasteéis, 1940) which I will discuss, epiboly more
especially.
The passage from the static state of premorphogenesis to a dynamic
state is the essential fact in gastrulation. Actually, the possibilities of
gastrulation movements—spreading, cellular movements, etc.—exist
in a potential state in the pregastrula-stages, but here they are only
revealed by experimental interventions.
Gastrulation awakens these latent properties of the blastomeres and
of the cortex and co-ordinates them at a precise moment in the egg's
history. It would be tempting, and too simple, to account for this sudden
mobilization by the action of a gastrulation 'hormone'. This conception
comes up against the fact that isolated structures cultivated in vitro
show some of the gastrulation movements (Holtfreter, 1943a, b, c, on
the egg of Amphibia). The enveloping layer, when cultivated alone,
shows its capacity for epiboly (Devillers et al, 1957b). Here we have a
mysterious phenomenon of 'cytoplasmic maturation' which affects at
the same time all the structures of the egg.
A. Epiboly
The mechanism of epiboly is the result of a co-operation of several
systems of forces localized in the egg covering, the perivitelline pellicle
(cortex) and the syncytium (Devillers, 1948b, c, 1950a, b, 1951a, b,
1952; Devillers et al, 1957b; Trinkaus, 1949a, b, 1951, 1952a, b;
Trinkaus and Gilmartin, 1949) (Fig. 14).
1. Part Played by the Enveloping Layer
Sections of blastoderms of Salmo at different stages in epiboly show
that this movement concerns especially the enveloping layer: its cells
spread out and become thinner. Cellular multiplication does not seem to
be essential and activity of the deep blastomeres is in no way apparent:
they do not change shape but only rearrange themselves to compensate
for the increase of the surface of the germ: the roof of a blástula has five
to six layers of deep blastomeres while only two remain in a neurula. On
the other hand, one can produce experimentally (Luther, 1935), or discover in a natural state, germs that are very poor in deep blastomeres
and in which epiboly occurs normally (without formation of an embryo).
The spreading of the enveloping layer is an autonomous movement,
and not a simple stretching caused by traction of the perivitelline pellicle,
as supposed by Lewis (1949a, b). Thus, the blastoderm of Fundulus
407
achieved and the vitelline syncytium is formed; polarity and symmetry
are fixed and the marginal zone is determined.
Since the egg is isolated from the medium by its impermeable cortex,
intrinsic factors must start the morphogenetic movements characteristic
of the vertebrate egg (Pasteéis, 1940) which I will discuss, epiboly more
especially.
The passage from the static state of premorphogenesis to a dynamic
state is the essential fact in gastrulation. Actually, the possibilities of
gastrulation movements—spreading, cellular movements, etc.—exist
in a potential state in the pregastrula-stages, but here they are only
revealed by experimental interventions.
Gastrulation awakens these latent properties of the blastomeres and
of the cortex and co-ordinates them at a precise moment in the egg's
history. It would be tempting, and too simple, to account for this sudden
mobilization by the action of a gastrulation 'hormone'. This conception
comes up against the fact that isolated structures cultivated in vitro
show some of the gastrulation movements (Holtfreter, 1943a, b, c, on
the egg of Amphibia). The enveloping layer, when cultivated alone,
shows its capacity for epiboly (Devillers et al, 1957b). Here we have a
mysterious phenomenon of 'cytoplasmic maturation' which affects at
the same time all the structures of the egg.
A. Epiboly
The mechanism of epiboly is the result of a co-operation of several
systems of forces localized in the egg covering, the perivitelline pellicle
(cortex) and the syncytium (Devillers, 1948b, c, 1950a, b, 1951a, b,
1952; Devillers et al, 1957b; Trinkaus, 1949a, b, 1951, 1952a, b;
Trinkaus and Gilmartin, 1949) (Fig. 14).
1. Part Played by the Enveloping Layer
Sections of blastoderms of Salmo at different stages in epiboly show
that this movement concerns especially the enveloping layer: its cells
spread out and become thinner. Cellular multiplication does not seem to
be essential and activity of the deep blastomeres is in no way apparent:
they do not change shape but only rearrange themselves to compensate
for the increase of the surface of the germ: the roof of a blástula has five
to six layers of deep blastomeres while only two remain in a neurula. On
the other hand, one can produce experimentally (Luther, 1935), or discover in a natural state, germs that are very poor in deep blastomeres
and in which epiboly occurs normally (without formation of an embryo).
The spreading of the enveloping layer is an autonomous movement,
and not a simple stretching caused by traction of the perivitelline pellicle,
as supposed by Lewis (1949a, b). Thus, the blastoderm of Fundulus