GERM LAYERS. 



61 



Gastrulation in Reptiles and Birds. 



This is further modified by the still greater increase in yolk, yet retains 

 sufficient similarity to the process in Amphibians and Amphioxus to allow of 

 comparison. 



Fig. 40. — Surface view of blastoderm of snake. Hcrtwig. Blastopore is represented by dark 

 transverse band near lower side of figure. 



In the types of gastrulation thus far described— in Amphioxus, Triton and 

 the frog — the entire egg is involved in segmentation and gastrulation. Up 

 through these forms there is a progressive increase in the amount of yolk, which 



Embryonic disk 



Blastoderm 



Anterior lip 



Posterior lip 

 of blastopore 



Blastopore 

 (crescentic groove) 



Fig. 41. — Surface view of embryonic disk of turtle (Emys taurica). Bonnet. 

 X, The lighter shading represents the opacity due to the growth of the protentoderm (see Fig. 42). 



in Triton and still more in the frog was seen to modify the gastrulation process. 

 In the reptilian and the avian ovum there is a much greater increase in yolk 

 content, the segmentation being confined to the germ disk and to a small part of 



