268 



ANIMAL BIOLOGY 



Gray crescent 



Animal 

 pole 



B 



Vegetal pole 



Blastocoel 



D 



Yolk cells 



Enteron 



E 



G 



Gill arches 



Yolk plu 

 Blastopore 



Endoderm 



Ectoderm 

 Blcl. 



Neural groove 



Notochord 

 Neurenteric 

 canal 



Anal / 

 region 

 Rectum 



Yolk 



Ectoderm 



Yolk plug 



Spinal cord 



Midbrain 

 Forebrain 



Enteron 

 Sucker plate 

 Endoderm 

 Mesoderm 



Fig. 174. — Early development of the Frog. A, B, C, egg at two-, four-, 

 and eight-cell stage; D, early blastula, E, section of D; F, late blast ula; G, early 

 gastrula: overgrowth of ectoderm (cells of ectoderm now too small to be 

 shown in figure); H, section of G showing germ layers, etc. (Blcl, blastocoel); 

 I, late gastrula : formation of neural groove and folds representing the founda- 

 tions of the nervous system; J, older embryo, with neural groove closed, as- 

 suming tadpole form; K, section of J. See Fig. 175. 



