230 



ZOOLOGY. 



find that later on in the frog's development the archenteron 

 (or rather mesenteron) is formed in this irregular mass of 

 yolk-cells by a process of splitting, or separation of the cells 

 from one another. So far, then, the differences in develop- 

 ment in our two types are all directly explicable by reference 

 to the increased amount of yolk in the frog. 



c D 



Fig. 118. DEVELOPMENT OF EMBRYO FROG, 

 (After Marshall.) 



The next stages in Amphioxus (fig. 113, c) showed the 

 obliteration of the segmentation cavity by more complete 

 invagination, and the restriction of the orifice of invagina- 

 tion to a small opening, the blastopore the epiblast coming 

 to form the whole outer surface except at the blastopore 

 itself. In the frog the primitive epiblast cells are not 



