GERM LAYERS. 



71 



Embryonic disk 

 Hensen's node 



Primitive streak 



and groove 



Embryonic disk _ 



Completion plate 



f Head process 



. Prim. int. cord 

 (protentoderm) 



> Mesoderm 



Ectoderm 







Yolk entoderm 



Ectoderm 



Mesoderm 



Mesoderm 



Completion plate 



Medullary folds 



r - .&i$$ 



^^^^^^^^^ rnmitive groove 



f*^^*****^#f^;*jy^^^ffirarjty i3 .. j ..Vi'iJ*/***'' 



'**..^ ^^ imm *t** *^.* 



S< 



Mesoderm 



Yolk entoderm 



Post, end of prim, streak 



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 (S r -S r ) indicate planes 

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



