no 



COMPARATIVE ANATOMY 



amphibian and bird at similar stages and at corresponding planes may 

 appear to be so unlike as to be hardly comparable. Yet careful study 

 of them reveals that the differences are mainly quantitative. The}^ are 



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



GASTRULATION IN FORM WITH TELOLECITHAL EGG CONTAINING MODERATE 

 AMOUNT OF YOLK— AMPHIBIA. 



gastrocoele 



GASTRULATION IN FORM WITH TELOLECITHAL EGG CONTAINING LARGE 

 AMOUNT OF YOLK— BIRDS. 



Fig. 8i. — Diagrams illustrating the differences in gastrulation depending on whether 

 the egg is isolecithal (having little yolk uniformly distributed) or telolecithal (having 

 much yolk more or less concentrated toward one pole of the egg), blc, blastocoele; 

 bid., blastoderm; blp., blastopore; ect., ectoderm; enl., entoderm; mil., cell undergoing 

 mitosis; yk., yolk; yk.g., cells containing yolk granules; yk.p., yolk plug. (From Patten, 

 Embryology of the Chick.) 



differences in proportions and extent of layers. The essential relations 

 of layers and developing organs are the same. View a mid-trunk trans- 

 verse section of an early frog embryo and imagine that the mass of yolk 



