22 



BIOLOGY OF THE LABORATORY MOUSE 



Primitive streak - 



Entoderm 



Mesoderm 



Ectoderm 

 Head process 

 Squamous entoderm 



Mesoderm 



Head process 



Squamous 

 entoderm 



3. Aohcn 

 Notochord 



Fig. 14.— For descriptive legend see opposite page. 



