EGG CORTEX IN MORPHOGENESIS
375
Case (3) has been already considered in the preceding section (see
experiments on the eggs of the axolotl). Experiments made with Rana
temporaria
eggs lead to the same conclusions (Pasteels, 1940a) and
emphasize clearly the action of the cortical field, which seems to be
unaffected.
Case (1) corresponds to the original experiment of Born which has
been later carefully analysed by Ancel and Vintemberger (1948). The
FIG. 8. Effects of the partial turning (135°) of an egg of Rana temporaria around an
axis parallel to the plane of symmetry, (a) Beginning of the experiment : the vegetal yolk
flows in the direction of the arrow; the densely dotted area α represents the grey crescent.
(b) Result of the flow of yolk inside the egg: Born's crescent at the point ß. (c) Asymmetric
embryo resulting when the dorsal blastopore lip has appeared at β (Bom's crescent), the
grey crescent (a) being at its right side.
downward flow of the yolk provokes, at the side which has been placed
in the upper position, the formation of a clear crescent that corresponds
completely with the future grey crescent appearing at the same place.
As Ancel and Vintemberger have shown, the rotation of the egg to a
forced oblique position is the most powerful 'preparing factor' of
sy mmetr ization.
We shall consider case (2) more closely. Eggs with the grey crescent
already established are turned in such a way that the flow of the heavy
yolk is perpendicular to the plane of symmetry (Fig. 8). Born's crescent
(β) appears perpendicular to the pre-existing grey crescent (a). Such
375
Case (3) has been already considered in the preceding section (see
experiments on the eggs of the axolotl). Experiments made with Rana
temporaria
eggs lead to the same conclusions (Pasteels, 1940a) and
emphasize clearly the action of the cortical field, which seems to be
unaffected.
Case (1) corresponds to the original experiment of Born which has
been later carefully analysed by Ancel and Vintemberger (1948). The
FIG. 8. Effects of the partial turning (135°) of an egg of Rana temporaria around an
axis parallel to the plane of symmetry, (a) Beginning of the experiment : the vegetal yolk
flows in the direction of the arrow; the densely dotted area α represents the grey crescent.
(b) Result of the flow of yolk inside the egg: Born's crescent at the point ß. (c) Asymmetric
embryo resulting when the dorsal blastopore lip has appeared at β (Bom's crescent), the
grey crescent (a) being at its right side.
downward flow of the yolk provokes, at the side which has been placed
in the upper position, the formation of a clear crescent that corresponds
completely with the future grey crescent appearing at the same place.
As Ancel and Vintemberger have shown, the rotation of the egg to a
forced oblique position is the most powerful 'preparing factor' of
sy mmetr ization.
We shall consider case (2) more closely. Eggs with the grey crescent
already established are turned in such a way that the flow of the heavy
yolk is perpendicular to the plane of symmetry (Fig. 8). Born's crescent
(β) appears perpendicular to the pre-existing grey crescent (a). Such