12 



BIOLOGY OF THE LABORATORY MOUSE 



.Embryonic ectoderm 



B. Bohe.n 



Fig. 9. — Diagram comparing the eariy stages of development in a primitive rodent, 

 the thirteen-striped ground squirrel, and in the mouse. A, B and C. The thirteen- 

 striped ground squirrel. {After Mossman and Weisfeldt.) D, E and F. The mouse 

 {Mus muscidus). (A and B, X24; C, X8; D and E, Xioo; F, X50.) 



