Facts and Factors of Development 

 A B 



29 



FIG. 14. A, human embryo of forty-two somites, age twenty-one days; 

 B, embryo of about four weeks; C, still older embryo showing the begin- 

 nings of the formation of digits; D, embryo of about two months; C and 

 D are drawn on a smaller scale than A and B. (After.Keibel.) 



