FIG. 4. Schematic drawing explaining the different possibilities in the relations
between vegetal yolk and cortex in inverted eggs and the localization of dorsal lips.
Upper left: before the experiment; upper right: just after the experiment. The four other
figures represent cases at gastrulation. The asterisks indicate the localization of the
central point of the grey crescent before the experiment. An. : animal pole ; Veg. : vegetal
pole ; bl. : dorsal blastoporal lip.