40 THE BREEDING OF ANIMALS 



can actually be observed in the cell during the fertiliza- 

 tion and maturation of the germ-cells than any other so 

 far advanced. 



39. The origin of the germ-cells. The origin of the 

 germ-cells and the phenomena attending the process of 

 the reduction of the chromosomes are of great fundamental 

 significance. The functions of the chromatin in modern 

 theories of heredity, the particular meaning of the pro- 

 cesses which result in reducing the number of chromosomes 

 to one-half that found in the body-cells, are problems 

 of the greatest interest in modern biology. 



The maturation of the germ-cells is brought about 

 by similar processes in the egg and sperm. The impor- 

 tant result which is the reduction of the number of chro- 

 mosomes is accomplished in each. This is brought about 

 finally in the last two maturation divisions resulting in four 

 cells, each of which contains but one-half the number of 

 chromosomes characteristic of the same cells. In the 

 female three of the four cells are the polar bodies which 

 are abortive and disappear. The remaining cell is the 

 ovum and becomes the carrier of the hereditary substance 

 of the female. In the male the reduction divisions occur 

 as in the female, but all four cells are functional and may 

 take part in the process of fertilization. 



40. Maturation and reduction in the female (oogenesis) . 

 The female germ-cells are derived from the primordial 

 germ-cells of the mother. Successive divisions of the 

 primordial germ-cells result in the development of a 

 number of cells known as oogonia. 



From these the ovarian eggs are directly derived. 

 Their growth is characterized by an increase in the size 

 of the nucleus which in the ovarian egg (oocyte) becomes 

 the germinal vesicle. Food materials develop in the 



