TJie Method of Evolution 



47 



initiate specific metabolic processes in a suitable medium 

 represented by the larger portion of the egg. 



In egg and sperm, before their union, the chromosome 

 number is reduced one-half, from the double to the single 

 condition; in Ascaris, from 2 to i ; in the sea-urchin, from 

 36 to iS; in the mouse and in the man from 24 to 12. 

 These reductions occur in what is called the maturation 

 of the sexual products. 



In the male, the primitive germ cell containing the 

 double or 2N number of 

 chromosomes divides up 

 into a group of four cells, 

 each containing the single or 

 A^ number of chromosomes. 

 This comes about by a fail- 

 ure of the chromosomes to 

 split at one of the two cell 

 divisions which produce the 

 group of four sperm cells, 

 as they regularly do in 

 ordinary cell division. A 

 tadpole-like sperm now arises from each of the cells con- 

 taining the reduced number of chromosomes. 



In the maturation of the egg, reduction is likewise 

 accomplished by two cell divisions, in one of which the 

 chromosomes do not split as in ordinary cell divisions. 

 The divisions of the egg, however, are into parts of very 

 unequal size, only one of which is fertilized, the rest failing 

 to develop. For example, in the marine worm Xercis, 

 according to Wilson, the maturation of the egg occurs 

 simultaneously with its fertilization. The first maturation 

 division separates off a minute cell, known as the first polar 



Fig. 20. — Diagram to explain the result 

 shown in Figs. 16-19. 



