loo Vertebrate Embryology 



its division migrate to the superficial layer of 

 protoplasm which then divides into as many 

 segments as there are daughter nuclei. In this 

 way a germ-membrane is formed around the 

 outside of the egg. This form of cleavage is 

 illustrated by many Arthropods. 



At the close of segmentation the egg, as 

 was described in connection with the frog, is 

 converted into a hollow sphere known as the 

 blastula. 



By a process of invagination the blastula is 

 converted into a gastrula, a two-walled sac, 

 opening to the exterior by the blastopore. As 

 the blastula is the result of the process of seg- 

 mentation, its form will be dependent upon the 

 amount and distribution of the food yolk ; and 

 in like manner the character of the gastrula- 

 tion will also depend upon the yolk. 



In some eggs gastrulation is so plain and 

 evident that it may easily be made out, but 

 in other eggs it is so masked by the large 

 amount of food yolk that it is very difficult to 

 determine. 



Four kinds of gastrulse are sometimes de- 

 scribed : 



I. Where the egg is small and free from 

 yolk, as in Amphioxus, the archenteron is wide, 



