MATURATION 



269 



egg + ^perra a fertile egg 



double were it not for certain changes which occur in the de- 

 velopment of the egg and sperm. These changes are called mat- 

 uration or the ripening of the 

 gametes. The cells that are to 

 produce the germ cells or gam- 

 etes are the primary sex cells. 

 They have the same number of 



chromosomes as the Somatic Or Fertilization is the union of dissimilar gametes ; 



body cells. When ready to ma- theunio 



ture, the sex cells split by a process much like mitosis, but the chro- 

 mosomes do not split lengthwise. Instead of splitting evenly, 

 one half the total number of unsplit chromosomes goes into one 

 daughter cell and one half goes into the other. This is the part 

 of the ripening or maturation process called reduction division. 



The number of chromo- 

 somes is reduced one 

 half. After this, each of 

 these cells again divides, 

 mitotically. Thus from 

 every primary sex cell, 

 cells are produced, each 

 of which has half the 

 original number of chro- 

 mosomes. If. the pri- 

 mary sex cell had eight 

 chromosomes, each 

 gamete would have half 

 this number or four chro- 

 mosomes. The produc- 

 tion of the sperms by 

 maturation of the pri- 

 mary sex cells is called 

 spermatogenesis; the 



The primary sex cells of the fruit fly have eight chro- 

 mosomes. During the ripening process, reduction divi- 

 sion takes place; the resulting cells have one half the 

 original number, in this case four chromosomes. Each 

 of these cells then divides, mitotically forming the sex 

 cells. From each primary male sex cell there forms four 

 sperms, all of which can function. Each primary female 

 sex cell gives rise to one egg and three reduced cells 

 known as polar cells. The egg has most of the yolk and 

 is generally the only one of the female sex cells to function 

 as a gamete. One sperm unites with a mature egg in the 

 process of fertilization. 



