SEX AND THE INDIVIDUAL 185 



is impregnated by the sperm cell, the cell undergoes 

 a remarkable change, in which the nucleus is stimu- 

 lated to new activity and to the process of division 

 which results in the formation of two cells. This 

 division marks the beginning of the embryo. In the 

 cell divisions that now follow in rapid succession it is 

 the nuclei that are especially prominent, both because 

 the divisions (or mitoses, as they are technically 

 called) have their starting point in the nuclei and 

 because the nuclear substance is now very abundant 

 as compared with the surrounding protoplasm. 

 The dominance of the nuclear substance during 

 growth is a characteristic of this period of life. 

 Gradually, as the rapidity of cell division dimin- 

 ishes and growth becomes less, the cell nucleus comes 

 to form a small part of the entire cell, and when 

 maturity is reached and cell division is rare, it is the 

 cell protoplasm that dominates. Hence, it appears 

 that this period of development is the period of 

 nuclear dominance and activity, while the period 

 of maturity is one of lesser or altered nuclear active 

 and reactive dominance of the cell protoplasm. The 

 important thing to realize here is that the adult 

 cells, in the course of their development and differ- 

 entiation, have largely surrendered their powers of 

 reproduction. New powers are, however, acquired 

 by these adult cells. The youthful and aggressive 

 reproductive functions of the young cells have been 

 exchanged for the more stable, more diversified, 

 and more elaborately coordinated functions of the 



