126 THE STORY OF LIFE'S MECHANISM. 



with just one half the number of chromosomes 

 normal for the cells of the animal in question. 

 Meantime the first pole cell has also divided, so 

 that we have now, as shown in Fig. 40, four 

 cells, three small and one large, but each con- 

 taining one half the normal number of chromo- 

 somes. In the example figured, four is the 

 normal number for the cells of the animal. The 

 egg at the beginning of the process contained 

 eight, but has now been reduced to two. In the 

 further history of the egg the smaller cells, 

 called polar cells, take no part, since they soon 

 disappear and have nothing to do with the 

 animal which is to result from the further 

 division of the egg. This process of the for- 

 mation of the polar cells is thus simply a device 

 for getting rid of some of the chromatin material 

 in the egg cell, so that it may unite with a 

 second cell without doubling the normal number 

 of chromosomes. 



Previously to this process the other sexual 

 cell, the spermatozoon, or male reproductive cell, 

 has been undergoing a somewhat similar process. 

 This is also a true cell (Fig. 34, me), although 

 it is of a decidedly smaller size than the egg 

 and of a very different shape. It contains cell 

 substance, a nucleus with chromosomes, and a 

 centrosome, the number of chromosomes, as 

 shown later, being however only half that 

 normal for the ordinary cells of the animals. 

 The study of the development of the sperma- 

 tozoon shows that it has come from cells which 

 contained the normal number of four, but that 

 this number has been reduced to one half by a 



