20 



TEXT-BOOK OF EMBRYOLOGY. 



Thomson's. In an extensive series of ovaries he has observed both polar 

 bodies and the spindles preceding extrusion. Both maturation divisions 

 occur before the Graafian follicle ruptures and discharges the ovum, the 

 time of formation of the second polar body therefore differing from that in 

 other mammals. 



From the data in the above description it is evident that the phenomena 

 of maturation are essentially similar in the male and female sex cells. In 

 the female two or three of the cells are indeed abortive, probably in order to 

 insure a large amount of food material to the functioning ovum; but the 

 result, the reduction of the number of chromosomes in the mature sex cell 



Oogonia 



Primary 

 oocyte 



Secondary 

 oocyte 



Spermatogonia 



Proliferation 



Primary 

 spermatocyte 



Growth 



Secondary 

 spermatoeyte 



Spermatid 

 Maturation 

 Spermatozoon 



j 



[if! 



Prolifera- 

 tion 



Growth 



Maturation 



Trans- 

 formation 



FIG. 12. Diagram representing the histogenesis of (a) the female sex cells and (6) the male sex 



cells. Modified from Boveri. 



to one-half the number characteristic of other cells of the species, is always 

 the same. 



Significance of Mitosis and Maturation. 



The earlier investigators regarded maturation merely as a means of re- 

 ducing the number of chromosomes in the mature germ cells, so as to prevent 

 a doubling of chromatin material at the subsequent fertilization. This, 

 however, seems to be but a minor object of maturation. As a matter of 

 fact, the reduction of the chromatin mass is not one-half but three-quarters 

 and even more. It is also well known that the chromatin mass increases or 

 diminishes under certain conditions during the life history of a cell. 



The chief significance of maturation is to be considered rather from the 

 standpoint of heredity. Modern biologists are convinced that the chromatin 

 particles constitute the inheritance substance of the cell. During mitosis 



