9 6 



THE BIOLOGY OF THE FROG 



thick. The outer of these layers is the ectoderm ; the inner, 

 the entoderm. The cells of the former are small and pig- 

 mented ; those of the latter for the most part are compara- 

 tively large, lighter in color, and contain a large amount of 

 yolk. The two layers are continuous with each other at the 

 lips of the blastopore. Before the process of invagination 



Y DP' 



FIG. 20. — Sagittal section through a frog embryo. B, blastoccel ; BP, 

 dorsal lip of blastopore ; BP' , ventral lip of blastopore; EE, epidermic 

 layer of ectoderm ; EN, inner or nervous layer of ectoderm; //.hypo- 

 blast or entoderm; T, mesenteron or gastrula cavity; Y, yolk plug. 

 (After Marshall.) 



is completed there appears a third germ layer, the mesoderm 

 or mesoblast, between the other two. The mesoderm ap- 

 pears all around the blastopore, and as this opening closes 

 mainly from in front backward, the two masses of meso- 

 derm on either side are brought near each other in the mid- 

 dorsal line. The free ventral edges of the masses or sheets 



