214 



ESSENTIALS OF ZOOLOGY 



which is now a single layer. The formation of the blastula, which 

 usually comes at the sixty-four-cell stage or later, marks the end 

 of cleavage. The blastula stage of an animal like a starfish or a 

 frog resembles somewhat a hollow rubber ball whose wall is made 

 up of a large number of pieces cemented together. 



nut. 



bid. 



GASTRULATION IN FORM WITH ISOLECITHAL EGG HAVING ALMOST NO YOLK— AMPHIOXUS. 



ea. 



:— - blp. 



GASTRULATION IN FORM WITH TELOLECITHAL EGG CONTAINING MODERATE 



AMOyNT OF YOLK— AMPHIBIA. 



ent 



gastrocoele 



GASTRULATION IN FORM WITH TELOLECITHAL EGG CONTAINING LARGE 



AMOUNT OF YOLK— BIRDS. 



Fig- 63.— Gastrulation in three types of embryos. Each type begins with the late 

 blastula at the left, blc, blastocoele ; bid., blastoderm; bZp., blastopore; ect., ecto- 

 derm; ent., endoderm; mit, cells undergoing mitosis. (From Patten. Emhryoloay 

 of the Chick, by permission of P. Blakiston's Son and Company.) 



