58 



TEXT-BOOK OF EMBRYOLOGY. 



the underlying yolk (p. 56). Just as cleavage in Reptiles and Birds was 

 modified by the presence of the large unsegmenting yolk mass, so, for the same 



Ectoderm of embryonic disk 



Blastopore 



Ectoderm 



Yolk entoderm 

 Blastopore 



Ectoderm 



'Completion 

 plate" 



Protentoderm 



Yolk entoderm 



Blastopore 



Peristomal mesoderm 



Peristomal 

 Blastopore mesoderm 



"Completion 

 plate" 



Remnant of 

 protentoderm 



Blastopore 



Peristomal 

 mesoderm 



"Completion plate" Yolk entoderm 



FIG. 42. From medial vertical sections through embryonic disk of lizard, showing five successive 

 stages in gastrulation. Wenckebach, Bonnet. 



reason, is gastrulation quite modified, as compared with the simple process seen 

 in Amphioxus. At the same time, however, it is possible to correlate the reptil- 

 ian and avian gastrulation with gastrulation in the lower forms. 



