EARLY MAMMALIAN DEVELOPMENT. 



97 



Embryonic disk 

 Hensen's node 



Primitive streak 



and groove 



Embryonic disk 



Head process 



_. Prim. int. cord 

 (protentoderm) 



Mesoderm 



Ectoderm 







Mesoderm 



Yolk entod 



Ectoderm 





Chordal plate (prol 

 Primitive groo\ 



Mesoderm 



1 Mesoderm 



Mesoderm 



Mesoderm 



Yolk entoderm 



FIG. 70. Surface view of embryonic disk of dog and transverse sections of same. Bonnet. 

 a, Disk somewhat further advanced than that in Fig. 64; the letters and figures (Si-S 6 ) indicate 



planes of sections in b; rn.gr., medullary groove. 

 7 



