SEX INHERITANCE 137 



in Lychnis the male-producing pollen do not fertilize 

 proportionately as many eggs in competition as they 

 do when not enough pollen is present to fertilize 

 all the eggs. Presumably the pollen-tubes do not 

 grow as fast as those of the female-producing pollen. 



There is one situation that calls for special notice. 

 In the WZ — ZZ type (moths and birds) the egg 

 contains WZ in conjugation before the polar bodies 

 are extruded. If it is only a matter of chance 

 which way this pair (WZ) lies on the spindle, then 

 equal numbers of W^-eggs and Z-eggs result after 

 the polar bodies have been given off. But if the 

 WZ pair should be placed so that the Z went out 

 more often, more females would be expected; and 

 if the W Avent out more often, then more males 

 would result. Now in a few cases (Doncaster and 

 Seller) the W-chromosome is absent. In the egg 

 of such a female there would be a ''lagging" Z on 

 the maturation spindle and if this tended to pass 

 out (or to be lost) more often than to remain in 

 the egg, the sex-ratio would be turned toward 

 females. In fact any sex-ratio might result from the 

 OZ type of individual. Moreover, once started, 

 such an OZ line would perpetuate itself. In the 

 case of the Y chromosome of the XX — XY type it 

 makes no difference in the sex-ratio to which ])ole 

 the X goes since botli polen ]:)roduce functional 

 sperm, and the Y has disappeared in many cases. 



Another way in wliicli sex-ratios are influenced is 

 through the viability of tlie zygotes. In extreme 

 cases, occurring in Drosophila all the offspring of a 



