CLASS AMPHIBIA 



535 



the others; and the rim on this side, or the dorsal lip, is forced down 

 to the vegetal pole and a little beyond on the other side before gas- 

 trulation is complete. The area enclosed by the circle or blastopore 

 is finally very small, and the white vegetal cells which fill it are called 

 the yolk ping. 



While the animal cells have been advancing and covering the 

 vegetal cells, changes have been going on internally in the eg,g. The 

 appearance of the groove or dorsal lip of the blastopore was caused 



Ectoderm — 



Endoderm-- 



Arch(?nfccron 

 or 



primitive 



intestine 



dorsal Up of blastopore'' 



Yolk plaq-i -Ventral lip of bIa5topore 



Fig. 289. — Gastrula stage of developing frog in section. 



by an infolding of outside cells. This ingrowth of cells progresses 

 around the egg as the crescent groove extends itself. The cells 

 which consequently come to lie inside are known as endodermal 

 cells, those on the outside as ectodermal cells. 



By this invagination a cavity known as the archenteron is formed, 

 the walls of which are made up of the invaginated endoderm cells. 

 At first it is quite flat, but it expands as invagination and the other 



