GERM LAYERS. 



71 





Embryonic disk 

 Hensen's node 



Primitive streak 



and groove 



Embryonic disk 



Completion plate 



> Head process 



Prim. int. cord 

 (protentoderm) 



Mesoderm 



Volk entoderm 



Completion plate 



Medullary folds 



Mesoderm 



S 2 



Mesoderm 



Mesoderm 



Yolk entoderm 



Yolk entoderm 



Chordal plate (protentoderm) 

 Primitive groove 



E**i!iiIJ 







***"** ^*- 4 ^* : *'^^ f 



Post, end of prim, streak 



S, 



Mesoderm 



Yolk entoderm 



FIG. 54. Surface view of embryonic disk of dog and transverse sections of same. Bonnet, 

 a, Disk somewhat further advanced than that in Fig. 53; the letters and figures (Si-S s ) indicate planes 

 of sections in b. m. gr., medullary groove. 



