1 62 THE GROWTH OF THE BRAIN. 



The determination of the number of neuroblasts 

 occurs so early in the history of the individual, and 

 under such uniform conditions, that it is very difficult 

 to regard the environment as possessed of much 

 power to cause variation in this respect, and for this 

 reason among members of the same race a high degree 

 of constancy in this character is to be anticipated. The 

 influence of the surrounding conditions becomes much 

 more effective during the later stages of development 

 that accompany the enlargements of the elements 

 already formed, and it is during this period that adap- 

 tive modifications may occur. 



