3 Heredity and Environment 



tive and destructive metabolism, (2) Reproduction, as shown in 

 the division of cells and cell constituents, (3) Irritability, or the 

 capacity of receiving and responding to stimuli. All these gen- 

 eral functions of living things are manifested by germ cells, .but 



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.) 



