48 Heredity and Eugenics 



body, and is quickly followed by a second likewise unequal 

 division, by which a second polar body is produced. The 

 number of chromosomes remaining in the egg nucleus is 

 now reduced to half that in the egg before maturation. A 

 sperm entering the egg has formed within it a second nuclear 

 body which, like that of the egg contains a reduced number 

 of chromosomes. By the union of these two nuclei a new 

 nucleus is formed which contains the double number, and 

 from this all cells of the new individual are directly derived. 

 In all such cells the double chromosome number is present. 

 Similar events take place in the maturation and fertilization 

 of animal eggs in general. 



Now, when the egg of a black guinea-pig is fertilized with 

 the sperm of a white one, or vice versa, protoplasmic con- 

 stituents unite which in one case are able to produce a 

 black coat, in the other a white one. These constituents, 

 whatever they are, evidently separate from each other and 

 pass into different cell products when the germ cells of the 

 cross-bred individual ripen. It seems natural to suppose 

 that the separation occurs at the reduction of the chromo- 

 somes from the double to the single condition. Half the 

 sperms, accordingly, of the cross-bred black individual bear 

 black, half white, none both; and the same is true of the 

 eggs (Fig. 20). Experiment proves conclusively that this is 

 so. Blackness and whiteness behave in crosses like indi- 

 visible units. They may be brought together repeatedly 

 in crosses, but always separate again at the maturation of 

 the gametes. We call them unit-characters. Black is a 

 positive unit (presence of black pigment), white its corre- 

 sponding negative (absence of black pigment). 



Other unit-characters are quite independent, in their 

 inheritance, of black and white. Thus, the coat of a 



