Heredity and Sex 



67 



in each of these products 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 

 (Fig. 33, upper row). Starting with the double or 2N 

 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 chro- 

 mosomes, N. From each of 

 these four cells arises a 

 tadpole-like sperm. Con- 

 sequently, when the sperm 

 enters the egg at fertiliza- 

 tion it brings in a group 

 of N chromosomes (in the 

 mouse apparently 12), 

 which, added to the egg contribution of N chromosomes, 

 brings the number hi the new organism again up to 2N (in 

 the mouse 24). 



Now, as regards the maturation of parthenogenetic eggs, 

 those which are to develop without 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) (see Fig. 34, 

 left column). The eggs are, so far as we can discover, all 

 of a single type. They undergo maturation in the manner 

 already described, the chromosomes being reduced to the 

 N or simplex number. The eggs of most animals, after 

 they have undergone reduction, are incapable of develop- 



FIG. 33. Top row, normal spermato- 

 genesis; lower row, spermatogenesis of 

 simplex male. 



