36 HEREDITY AND ENVIRONMENT 



the adult. From this egg cell arise by divi- 

 sion many cells which differ from one another 

 more and more as development proceeds, until 

 finally the adult animal results. A specific 

 type of development is due to a specific organ- 

 ization of the germ cells with which develop- 

 ment begins, but the earlier differentiations 

 of the egg are relatively few and simple as 

 compared with the bewildering complexities 

 of the adult, and the best way of xmderstand- 

 ing adult structures is to trace them back in 

 development to their simpler beginnings and 

 to study them in the process of becoming. 



7. Development of Functions. — The de- 

 velopment of functions goes hand in hand 

 with the development of structures; indeed 

 function and structure are merely different 

 aspects of one and the same thing, namely 

 organization. All the general functions of 

 living things are present in the germ cells, viz., 



(1) Constructive 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 general functions of living 



