14 MENDELIAN SEGREGATION 
mode of inheritance is explicable on the supposition 
that it lies in the fourth pair of chromosomes. 
For an adequate understanding of the inheritance 
Fic. 7.—Normal eyes of Drosophila a, a’. Eyeless b-d; b, b’ top and 
side view of head of fly without eyes; c, c’ right and left eyes of another fly; 
d, small eye on right side, none on left. 
of factors in the first group it will be necessary to 
consider the distribution of the sex chromosomes 
(Fig. 8). In the female of Drosophila there are two 
X chromosomes (XX). After the conjugation and 
