SEX 83 



insects, it is equally true of many other animals, appar- 

 ently including man. Recent work has revealed a num- 

 ber of cases to which the above description is not ap- 

 plicable, but the principle remains the same ; namely, 

 that sex is determined at the moment of fertilization, 

 by the number of ^-chromosomes, or sex-chromosomes, 

 which go into the zygote. 



The y, when present, seemed to have no function at 

 all; but C. B. Bridges has lately published an account 

 of certain cases in Drosophila which appear to show 

 otherwise. Owing to certain abnormalities in chromo- 

 some distribution, it was possible to produce males 

 without the y v/hich is normally present in that insect. 

 They were quite ordinary in appearance, but absolutely 

 sterile. Taking advantage of these same abnormalities, 

 it was found that zygotes with y or yy, but no x, and 

 also those with 3*, were unable to live. 



It thus appears, on the face of this evidence, that sex 

 is determined by the amount of a particular kind of 

 chromatin, which exists in a special chromosome. One 

 portion produces a male, two portions a female, while 

 three are incapable of development. Is a female, then, 

 all that a male is, and something more ? Hardly so, for 

 femaleness inhibits the development of male character- 

 istics. Gametically, the female may be a product of the 

 male determiner plus another, but in development the 

 characters of the one are obviously not added to the 

 characters of the other. 



The arrangement provides that exactly half the off- 

 spring shall be male, and half female, on the average. 

 The chances of an egg cell being fertilized by one or the 

 other sort of sperm are even. Why, then, are the sexes 

 not equal in number in all animals ? There are various 

 other factors entering into this matter ; the chances of 



