IX. DEVELOPMENT OF THE TELEOSTEAN EGG
417
example, if an old gastrula (with a large blastocoel) is placed in a
hypertonic solution, 4 hours later, the gertii having retracted, its diameter has shrunk to that of a blástula. The egg is then taken back to
running water; after 15 hours it has regained its earlier gastrula
diameter; after 26 hours, it has reached that of controls. The experiment
has lasted 30 hours during which the epiboly of controls has proceeded
without interruption. As for the treated eggs, they have only taken 11
hours (from the 15th to the 26th hour) to accomplish the same spreading.
A detergent such as Duponol G at a concentration of 0-5 mg per cent
allows one to obtain more delicate effects, by slowing down epiboly
only, without influencing the other morphogenetic movements: the
embryo elongates on a disc, the diameter of which is inadequate for
its stage of development and of which it occupies the whole length.
4. Co-ordination of Forces
We have acknowledged the existence of two groups of forces: cortical
(surface) and syncytial (deep). The cortical movement is an expansion
radiating from the animal pole in the cellular portion, and a contraction
towards the inferior pole in the undivided zone, that of the perivitelline
pellicle. The syncytial force is also a radiating expansion (Fig. 14).
More than the absolute value of one or the other of these forces, it is
their co-ordination that conditions the perfect realization of epiboly, as
shown by chronological combinations (Devillers, 1952). Two'lots of eggs
of the same laying were reared at 4° C and 12° C. After a few days the
former were at the morula stage (young eggs, J), the latter were advanced gastrulae (old eggs, A) (Fig. 19). On the yolk (V) of a gastrula
(VA) where epiboly is about to start, the germ (G) was replaced by a
morula (GJ). In this composite egg, epiboly will stop and only start
again when it begins in the 'young' controls. (Experiments on normal
eggs show that the trauma due to the operation cannot cause this delay.)
In the opposite combination of a gastrula (GA) on the yolk of a morula
(VJ) the germ cannot spread but the epiboly of the enveloping layer,
being an autonomous phenomenon, leads to the lengthening of the germ
perpendicularly to the surface. In the first combination GJ/VA, the
union of forces residing in the vitelline base (syncytial spreading + pellicular contraction) cannot overcome the resistance of the germ so long as
the enveloping layer is not 'ready' for epiboly. In the second combination, the spreading force of the enveloping layer cannot overcome the
resistance of the structures of the vitelline base, which have not yet
started their epiboly. When they are ready, they will still not be able to
spread since they will be opposed by the forces of invagination and extension of the marginal material in the germ during organization, these
forces working in the opposite direction.
417
example, if an old gastrula (with a large blastocoel) is placed in a
hypertonic solution, 4 hours later, the gertii having retracted, its diameter has shrunk to that of a blástula. The egg is then taken back to
running water; after 15 hours it has regained its earlier gastrula
diameter; after 26 hours, it has reached that of controls. The experiment
has lasted 30 hours during which the epiboly of controls has proceeded
without interruption. As for the treated eggs, they have only taken 11
hours (from the 15th to the 26th hour) to accomplish the same spreading.
A detergent such as Duponol G at a concentration of 0-5 mg per cent
allows one to obtain more delicate effects, by slowing down epiboly
only, without influencing the other morphogenetic movements: the
embryo elongates on a disc, the diameter of which is inadequate for
its stage of development and of which it occupies the whole length.
4. Co-ordination of Forces
We have acknowledged the existence of two groups of forces: cortical
(surface) and syncytial (deep). The cortical movement is an expansion
radiating from the animal pole in the cellular portion, and a contraction
towards the inferior pole in the undivided zone, that of the perivitelline
pellicle. The syncytial force is also a radiating expansion (Fig. 14).
More than the absolute value of one or the other of these forces, it is
their co-ordination that conditions the perfect realization of epiboly, as
shown by chronological combinations (Devillers, 1952). Two'lots of eggs
of the same laying were reared at 4° C and 12° C. After a few days the
former were at the morula stage (young eggs, J), the latter were advanced gastrulae (old eggs, A) (Fig. 19). On the yolk (V) of a gastrula
(VA) where epiboly is about to start, the germ (G) was replaced by a
morula (GJ). In this composite egg, epiboly will stop and only start
again when it begins in the 'young' controls. (Experiments on normal
eggs show that the trauma due to the operation cannot cause this delay.)
In the opposite combination of a gastrula (GA) on the yolk of a morula
(VJ) the germ cannot spread but the epiboly of the enveloping layer,
being an autonomous phenomenon, leads to the lengthening of the germ
perpendicularly to the surface. In the first combination GJ/VA, the
union of forces residing in the vitelline base (syncytial spreading + pellicular contraction) cannot overcome the resistance of the germ so long as
the enveloping layer is not 'ready' for epiboly. In the second combination, the spreading force of the enveloping layer cannot overcome the
resistance of the structures of the vitelline base, which have not yet
started their epiboly. When they are ready, they will still not be able to
spread since they will be opposed by the forces of invagination and extension of the marginal material in the germ during organization, these
forces working in the opposite direction.