26 



TEXT-BOOK OF EMBRYOLOGY. 



Comparing maturation in the male and female sex cells, it is to be noted 

 that the descendants of the primordial germ cells, the spermatogonia and the 

 ob'gonia, proliferate by ordinary mitotic division, with the preservation of the 

 somatic number of chromosomes, up to a certain definite period in their life 

 history. They then cease proliferating for a time and enter upon a period of 

 growth in size (Fig. 17). The results of this growth are the primary spermato- 

 cyte and the ovarian egg or primary oocyte. When, however, the primary 

 spermatocyte and the primary oocyte prepare for division, the nuclear reticulum 

 in each case resolves itself into one-half the somatic number of chromosomes, 



FIG. 17. Diagrams representing the histogenesis of (a) the female sex cells and (b) the male sex 



cells. Modified from Boveri. 



and this reduced number is given to each of the resulting secondary spermato- 

 cytes and secondary oocytes. 



There is, however, this marked peculiarity in the division of the primary 

 oocyte, in addition to the reduction in chromosomes, that while the di- 

 vision of the nuclear material (chromosomes) is equal, the division of the 

 cytoplasm is very unequal, most of the latter remaining in one cell, usually 

 designated the secondary oocyte proper. The other cell is of course small, 

 owing to its lack of cytoplasm, and is extruded from the oocyte proper as the 

 first polar body (Figs. 9, 12, 13 and 17). In the next division, that of the second- 

 ary oocyte, a similar condition obtains. Each resulting cell contains the re- 

 duced number of chromosomes, but one of the cells is large containing nearly 

 all the cytoplasm and is the mature egg cell, while the other is small owing to its 

 small amount of cytoplasm, and is extruded from the larger cell as the second 



