EGG CORTEX IN MORPHOGENESIS
381
presumptive orientation of cleavage-plane (and ligature) was ascertained
a posteriori,
i.e. through the result obtained. This was a somewhat
'circulus viciosus'.
In 1950, Dollander did recognize that, if correctly chosen, Triton eggs
have a true clear crescent, which has the same value as the grey
crescent of the frog. He was thus able not only to repeat the ligaturing
experiments after correct recognition of the orientation of the dorsoventral axis, but also when the clear crescent area had been vitally
stained. This precise technique allowed the author: (1) to ascertain that
it was possible for lateral, dorsal or ventral blastomeres to produce
normal embryos; (2) to compare the position of the dorsal lip that
appeared with the primitive orientation indicated by the clear crescent.
(a) "Lateral" blastomeres obtained by sagittal ligature both develop
into embryos, complete though somewhat deficient along the side of the
ligature. This result was of course expected. 'Dorsal' and Ventral'
blastomeres obtained by a frontal ligature could develop in very
FIG. 11. Separation by a ligature of a dorsal and a ventral blastomere in Triton,
(a) Before the experiment, the arched line representing the area of clear crescent;
(6) after the experiment; (c) at neurula stage. In this case only the ventral blastomere
has developed into an embryo. (After Dollander, 1950.)