SEX INHERITANCE 135 



random fertilization occurs, and that the resulting 

 zygotes are equally viable. There is evidence 

 bearing on all these possible sources of disturbances 

 of the normal sex-ratios. 



In the males of aphids and phylloxerans all of 

 the no-X gametes degenerate; in the male bee 

 three of the four gametes fail to develop; in the 

 hornet two of the four; in Hydatina one of the 

 primary spermatocytes gives rise to two female- 

 producing gametes, the other, which would give rise 

 to male-producing gametes, fails to divide and then 

 degenerates. In all of these cases fertilization 

 produces only females. The imperfect pollen grains 

 that are found in many plants, do not appear to 

 come under this heading, since they are not involved 

 in sex-determination. 



Much of the genetic work goes to show that 

 selective fertilization does not take place. There is 

 one set of direct observations, on the unisexual 

 mollusk, Cumingia, that shows that the first sperm 

 that meets the egg, head on, enters the egg and 

 fertilizes it. It has been suggested that under 

 environmental conditions unfavorable to the sper- 

 matozoa, one kind may be affected more seriously, or 

 sooner, than the other kind, and thus bring about a 

 selective result; but no direct evidence has ever 

 been brought forward to substantiate such a view. 

 That one class of spermatozoa may travel faster 

 and hence may more often succeed in reaching the 

 egg first is a suggestion that is perhaps less hypo- 

 thetical than the last. Zeleny and Faust's careful 



