3Q 



1 1 credit x and Environment 



live 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 







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

 /?, 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.) 



