174 



EMBRYOLOGICAL TYPES 



are darkly pigmented) are relatively free from yolk, and there- 

 fore divide faster than the larger light-coloured yolk-laden 



E F 



Fig. 74. — Formation and closure of the blastopore during gastrulation in 

 Rana, seen from below. (From Jenkinson.) 



In A the dorsal lip of the blastopore has just appeared ; in B the lateral 

 lips have extended, and they almost meet in C ; in D the ventral lip of the 

 blastopore (which is now a complete circle) has been formed ; and the 

 diameter of the blastopore decreases in E and F. 



cells of the vegetative hemisphere. One result of this is that 

 the animal-pole cells begin to grow down over the lighter- 

 coloured cells. This process starts by the formation of a lip 



