!64 CAUSES OF VARIATION 



facts occurring just previous to fertilization and by which the 

 number of chromosomes in both the male and female germ cells is 

 reduced to one half the usual or somatic number, so that their 

 union at fertilization restores the true number of chromosomes 

 typical of the race. Thus, if the somatic number of chromosomes 

 is sixteen, the number in the germ cells at fertilization will be 

 eight each, or sixteen after fusion of the nuclei. This process by 

 which the number of chromosomes is halved in the germ cell is 

 known as reduction, and is supposed to be the significant feature 

 of the maturation process by which the male and female germ 

 cells are prepared for union. 



Parallelism in the sexes. Maturation and its attendant phe- 

 nomena of reduction in the number of chromosomes is a subject 

 that must be considered separately in the male and the female, 

 and yet there exists a strange parallelism worthy of notice. To 

 quote Wilson J : 



Recent research has shown that maturation conforms to the same type 

 in both sexes. . . . Stated in the most general terms this parallel is as fol- 

 lows : In both sexes the final reduction in the number of chromosomes is 

 effected in the course of the last two cell divisions, or maturation divisions 

 [as they are called], by which the definitive germ cells arise, each of the 

 four cells thus formed having but half the usual number of chromosomes. 

 In the female but one of the four cells [resulting from the two maturation 

 divisions] forms the ovum proper, while the other three, known as the 

 polar bodies? are minute, rudimentary, and incapable of development. In 

 the male, on the other hand, all four of the cells become functional sper- 

 matozoa. This difference between the two sexes is probably due to the 

 physiological division of labor between the germ cells, the spermatozoa 

 being motile and very small, while the egg contains a large amount of 

 protoplasm and yolk, out of which the main mass of the embryonic body is 

 formed. In the male, therefore, all of the four cells may 3 become func- 

 tional ; in the female the functions of development have become restricted 

 to but one of the four, while the others have become rudimentary. 



1 Wilson, The Cell, p. 234. 



2 The author is here speaking specifically of reproduction in animals, as 

 plants do not form polar bodies. The difference in plant and animal reproduc- 

 tion is, however, more in form than in significance. 



8 The author says " may " become functional. He means by this that each of 

 the four cells (spermatozoa) arising from the last two divisions is capable of 

 fertilizing an ovum, while of the four cells arising from the last two divisions in 

 the female only one is capable of being fertilized. 



