THE FORMATION OF THE ECTODERM AND ENTODERM 



39 



rabbit, the entoderm is split from the under side of the germinal area and 

 the cells soon grow around the inside of the blastodermic vesicle, and form 

 an inner entodermal sac (Fig. 16). In Tarsius, a creature classed by Hubrecht 



Ectoderm of embryonic disc 



Blastopore 



Ectoderm 



Yolk entoderm 

 Blastopore 



Ectoderm 



' Completion plate ' 



Prolentoderm 



Yolk entoderm 

 Blastopore 



Peristomal mesoderm 



' Completion 

 plate " 



' Completion plate 



Peristomal 

 mesoderm 



"Completion plate " 



Yolk entoderm 



Fig. 20. — From medial vertical sections through embryonic disk of lizard, showing five successive stages 



in gastrulation (Wenckeback, Bonnet). 



with the Primates, the entoderm cells, after splitting off, do not grow around 

 the wall of the vesicle as in the rabbit, but soon form an entodermal sac 

 separated by a space from the troph-ectoderm layer. Just how the vesicle is 



