HEREDITY AND SEX 



product. Consequently, in each of these prod- 

 ucts the number of chromosomes is reduced to 

 half what it is in the cells of the parental body. 

 Thus in the egg of the mouse, by maturation, 

 the number of chromosomes becomes reduced 

 from about twenty-four to about twelve. 



Similar changes occur in the developing 

 sperm-cell (see Fig. 5). Starting with the 

 double or 2 N chromosome number, there are 

 formed by two nuclear divisions, with only one 

 splitting of chromosomes, four cells, each with 

 the reduced or simplex number of chromosomes, 

 N. Consequently, when the sperm enters the 

 egg at fertilization it brings in a group of N 

 chromosomes (in the mouse apparently twelve), 

 which, added to the egg-contribution of N 

 chromosomes, brings the number in the new 

 organism again up to 2 N (in the mouse twenty- 

 four). 



Now, as regards the maturation of partheno- 

 genetic eggs, those which are to develop with- 

 out having been fertilized, three categories of 

 cases deserve separate discussion. The simplest 

 of these in many respects is found among the 

 social hymenoptera (ants, bees, and wasps). 



161 



