54 STRUCTURE OF THE VERTEBRATES 



nerve cord develops by the dorsal and median growth of the 

 neural folds; but the frog differs from Amphioxus, and agrees 

 with all other vertebrates, in having the folds meet at the mid- 

 dorsal line before the neural plate breaks away from the epi- 

 dermal cells. 



2 Cells h Cells 8 Cells 



Cleavage Stages of Frog's Egg 



16 Cells 



32 Cells 



Blastula Beginning of Gastrulation 



Longitudinal Sections of Early Embryo 



Blastopore! 



Longitudinal Sections : Completion of Gastrulation 



Late Larval Stage 



Fig. 19. Embryology of the Frog. The diagrams show the holoblastic 



cleavage and gastrulation. Gastrulation is almost entirely an overgrowth 



of the dorsal pole cells. 



