STRUCTURE OF THE VERTEBRATES 



65 



process to the extreme limit, the ventral yolk being non-cellular 

 and serving only as a supply of food. 



At the beginning of gastrulation the blastula is a plate of 

 cells, with an inner thicker portion which gives rise to the em- 



FiG. 23. Adaptive Radiations of the Reptilia. A group of extinct reptiles 

 showing a few of the major specializations. 



