96 THEORY OF EVOLUTION 



whatevei' its function, has a common inheri- 

 tance. 



At an early stage in the development of the 

 animal certain cells are set apart to form the 

 organs of reproduction. In some animals these 

 cells can be identified early in the cleavage 

 (fig. 48). 



The reproductive cells are at first like all the 

 other cells in the body in that they contain a 

 full complement of chromosomes, half paternal 

 and half maternal in origin (fig. 49). They 

 divide as do the other cells of the body for a 

 long time (fig. 49, upper row). At each 

 division each chromosome splits lengthwise and 

 its halves migrate to opposite poles of the spin- 

 dle (fig. 49 c). 



But there comes a time when a new process 

 appears in the germ cells (fig 49 e-h). It is 

 essentially the same in the egg and in the sperm 

 cells. The discoveiy of this process we owe to 

 the laborious researches of many workers in 

 many countries. The list of their names is 

 long, and I shall not even attempt to repeat it. 

 The chromosomes come together in pairs (fig. 

 49 a) . Each maternal chromosome mates with 

 a paternal chromosome of the same kind. 



