NOTOCHORD 



ECTODERM 

 BLASTOCOEL 



GASTROCOEL 



Fig. 214. Gastrulation in the gymnophionan Amphibia and in the bony ganoid, Amia 

 calva. (A, B, C, after Brauer, 1897; D, E, after Dean, 1896.) Sections A-C through de- 

 veloping embryo of Hypogeophis alternans. (A) Middle gastrula, sagittal section. Ob- 

 serve that gastrocoel forms by a separation of the entodermal cells. Blastocoel forms 

 similarly through delamination of entoderm from the overlying epiblast and by spaces 

 which appear between the cells in situ. (B) Transverse section through late gastrula. 

 (C) Sagittal section through late gastrula. (D) Late gastrula of Ahiia. Mass of yolk 

 in center is uncleaved; cellular organization is progressing peripherally around yolk mass. 

 (E) Later gastrula of Amia. The blastopore is closing, but a large yolk mass still re- 

 mains uncleaved. 



444 



