__ _ -ap. 



ap.^_ 



Fig. 75. — The process of gastrulation in Rana as shown by sagittal sections. 



In A the dorsal lip of the blastopore bias just appeared, and it is accen- 

 tuated in B ; in C a definite ingrowth is visible resulting in the formation of 

 the archenteron : in D the ventral lip of the blastopore has appeared, and 

 the yolk-containing cells of the vegetative hemisphere project through the 

 now circular blastopore as the yolk-plug ; in E the archenteron has extended 

 greatly at the expense of the blastocoel, which in F is almost obliterated. 

 During gastrulation the yolk-cells are heaped up on the ventral side of the 

 archenteron, as a result of which the egg rotates until its original axis is more 

 or less horizontal. 



a, archenteron ; ap t animal pole ; b, blastoccel ; br, brain ; dl, dorsal 

 lip of blastopore ; in, mesoderm ; ?i, notochord ; nee, neurenteric canal ; 

 np, neuropore ; sc, spinal cord : vl, ventral lip of blastopore ; yc, yolk-cells; 

 yp, yolk-plug. 



