CH. LIX.] 



SPERMATOGENESIS 



873 



oocyte, and it will be convenient to postpone the histological details 

 until we come to the oocyte, where we shall see a diagram to represent 

 the process. 



The result is that the secondary spermatocyte and its descendants, 

 the spermatids and spermatozoa, have only half the number of 

 chromosomes characteristic of the species. The maturing of a 

 spermatozoon thus occurs within the seminiferous tubes. 



Oogenesis. This occurs in the same general lines as spermato- 

 genesis, but with some rather important differences of detail. In 



Germ cell 



Spermatogonia 



Period of growth 



Primary Spermatocyte 



--Secondary Spermatocytes 



---Spermatids 



Heterotype\ 

 mitosis and I 

 reduction of f 

 chromosomes / 

 to half the 

 usual number) 



A 



:.TTf, 



+ j % ^---Spermatozoa 



Fio. 560. Diagram to illustrate spermatogenesls. 



the first place, the changes in the ovary occur concurrently with 

 certain changes in the uterus which result in menstruation. A 

 second important difference is that the formation of the mature ovum 

 occurs after the ovarian ovum has left the ovary, and is on its journey 

 along the Fallopian tube to the uterus, previous to fertilisation. A 

 third difference is one of size, the ovum being considerably larger 

 than the spermatozoon. 



We will take the phenomenon known as menstruation first. On 

 the average once every four weeks the uterus becomes congested, 

 and its mucous membrane thickened; finally some of the blood- 



