The Developmenf of the Vertebrate Embryo 59 



themselves exteriorly up against the ectoderm to become the second layer 

 of skin and interiorly against the endoderm to become the outer lining of 

 the gut. Eventually internal organs such as the gonads, kidneys, etc. will 

 pinch off from the mesoderm and inhabit the internal body cavity that 

 lies between the mesodermal linings. With the formation of the third basal 

 layer, the gastrula is complete. 



THE FROG 



At the end of the cleavage stage, the frog embryo is a hollow ball 

 with a thin roof and a thick floor. A small dimple appears near the vegetal 

 pole when large yolky cells leave the surface of the egg and begin to 

 press inward. This process of inward movement is called invagination. 

 As more cells invaginate, the dimple becomes a crescent-shaped groove 

 curving downward toward the vegetal pole. The groove is called the 

 blastopore and has an upper ( dorsal ) and a lower ( ventral ) lip ( Fig. 29 ) . 



Fig. 29. Gastrulation stages in the frog. 



enclose orchenteron 



The sheet of cells from the upper surface of the blastula moves down- 

 ward and converges upon the blastopore. As the cells reach the dorsal lip, 

 they turn inward and enter the interior of the embryo. Similarly, the 

 ventral cells invaginate over the ventral lip of the blastopore. As the 

 presumptive ( future ) mesoderm folds inward over the dorsal lip, it sepa- 

 rates from the endoderm that entered before it. The mesoderm plasters 

 itself against the outer layer of ectoderm and continues to move away 



