COMPARATIVE EMBRYOLOGY 



805 



future animal. Bilateral cleavage is fairly widespread throughout 

 the animal kingdom, occurring in the Nematoda, Tunicata, amphioxus, 

 and in all the Vertebrata which have holoblastic cleavage ; that is, all 



^s- 



bid. 



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



PASTRULATION IN FORM WITH TELOLECITHAL EGG CONTAINING MODERATE 

 AMOUNT OF YOLK— AMPHIBIA. 



GASTRULATION IN FORM WITH TELOLECITHAL EGG CONTAINING LARGE- 

 AMOUNT OF YOLK— BIRDS. 



Fig. 416. — Gasti-ulation in three types of embryos. Typical discoidal blastula 

 formation (left) and gastrulatlon (riglit) in the telolecithal egg of the bird, below. 

 blc, blastocoele ; bid., blastoderm ; blp., blastopore ; ect., ectoderm ; ent., endoderm ; 

 riiit., cells undergoing mitosis. (From Patten, Embryology of the Chick, published 

 by permission of The Blakiston Co.) 



except the selachians, teleosts, reptiles, and birds, in which the cleav- 

 age is meroblastic, and the mammals, which are so specialized as not 



