THE DUALITY OF INHERITANCE 



equal contributions being made by egg and 

 sperm. This double condition persists through- 

 out the life of the new individual in all its 

 parts and tissues. But if the individual forms 

 eggs or sperm, these, before they can function 

 in the production of a new individual, must 

 undergo reduction to the single condition. 



This reduction process is called maturation; 

 it is well illustrated in the case of the mouse- 

 egg, whose fertilization has already been de- 

 scribed. The large nucleus of the egg-cell, as 

 it leaves the ovary, is either broken up or about 

 to break up preparatory to a cell-division. The 

 most conspicuous of the nuclear constituents 

 are some dense, heavily staining bodies called 

 chromosomes, about twenty-four in number. 

 In Fig. 3 each of these is split in two, prepara- 

 tory to the first maturation division. The egg 

 now divides twice, both times very unequally 

 (Fig. 4), forming thus two smaller cells called 

 polar cells, or polar bodies. They take no part 

 in the formation of the embryo. The chromo- 

 somes left in the egg after these two divisions 

 are only about half as numerous as before, or 

 about twelve in number. These form the chro- 



15 



