408
CH.
DEVILLERS
spreads over the syncytium even after the breaking of contact between
the enveloping layer and the pellicle (Trinkaus, 1951) (Fig. 18).
The elongation of the axial structures plays no part either, since an
embryo without chorda grows normally (Devillers, 1948b) and since
a blastoderm without an embryo undergoes epiboly. On the other
hand, some explants possessing all the axial structures stretch only
slightly.
A
EP
FIG. 14. Diagram of distribution of forces during epiboly in the egg of Salmo (from
Devillers, 1950b).
A: blástula stage; B: super-equatorial stage; C: sub-equatorial stage; CE: enveloping
layer; CS: latent force of contraction of the syncytial ridge; EP: spreading force of the
enveloping layer; ES: syncytial spreading force. In view C one sees that the action of the
syncytial contraction force can eventually add itself to that of the two others.
The existence in the enveloping layer of an intrinsic spreading force is
brought to light by cultivation in vitro of blastoderms (Oppenheimer,
1936; Tung et al, 1945; Devillers, 1947, 1949). At a stage which corresponds to the beginning of epiboly in control eggs, the explants stretch
and their surface increases, even if the mass of internal blastomeres is
not differentiated. Epibolic activity has started without any influence
from the rest of the egg, not even from the deep blastomeres. In fact if
the enveloping layer and the mass of deep blastomeres are cultivated
CH.
DEVILLERS
spreads over the syncytium even after the breaking of contact between
the enveloping layer and the pellicle (Trinkaus, 1951) (Fig. 18).
The elongation of the axial structures plays no part either, since an
embryo without chorda grows normally (Devillers, 1948b) and since
a blastoderm without an embryo undergoes epiboly. On the other
hand, some explants possessing all the axial structures stretch only
slightly.
A
EP
FIG. 14. Diagram of distribution of forces during epiboly in the egg of Salmo (from
Devillers, 1950b).
A: blástula stage; B: super-equatorial stage; C: sub-equatorial stage; CE: enveloping
layer; CS: latent force of contraction of the syncytial ridge; EP: spreading force of the
enveloping layer; ES: syncytial spreading force. In view C one sees that the action of the
syncytial contraction force can eventually add itself to that of the two others.
The existence in the enveloping layer of an intrinsic spreading force is
brought to light by cultivation in vitro of blastoderms (Oppenheimer,
1936; Tung et al, 1945; Devillers, 1947, 1949). At a stage which corresponds to the beginning of epiboly in control eggs, the explants stretch
and their surface increases, even if the mass of internal blastomeres is
not differentiated. Epibolic activity has started without any influence
from the rest of the egg, not even from the deep blastomeres. In fact if
the enveloping layer and the mass of deep blastomeres are cultivated