Embryologic Development of Animals 445 



Gray crescent 



Animal pole 



A 



B 



D 



Yolk cells 



Enteron 



Vegetal pole 



Blastocoel 



E 



Endoderm 

 Ectoderm 



Neural groove 





JR-EH 



Yolk plug 



Blastopore 



IT 



Neurenteric canal 



Notochord-^ 

 Blastopore 



Recttim 



Yolk 

 Ectoderm 



Yolk plug 

 Spinal cord 



Midbrain 

 Forebraiai 



Enteron 

 Sucker plats 

 Endoderm 

 Mesoderm 



Fig. 220. — Embryology of the frog. A, B, C, Two-, four-, and eight-cell stages 

 of dividing egg; D, early blastula; E, section of D; F, late blastula; G, early 

 gastrula with very small ectodermal cells overgrowing other cells; H, section of G, 

 showing germ layers, blastocoel {Bid), etc.; /, late gastrula with neural groove; 

 /, older gastrula with neural groove closed and assuming a tadpole form; K, sec- 

 tion of / (see also Fig. 221). (From Woodruff: Animal Biology. By permission 

 of The Macmillan Company, publishers.) 



