THE HEREDITY OF SEX 61 



but has obtained numerous mutations, so that his 

 work is a very important contribution to the muta- 

 tion doctrine. Drosophila in the hands of Professor 

 Morgan and his students and colleagues has thus 

 become as classical a type as Oenothera in those of 

 the botanical mutationists. Different branches of 

 Morgan's work are discussed elsewhere in this 

 volume, but here we are concerned only with its 

 bearing on the question of the determination of sex. 

 He describes ^ the chromosomes of Drosophila as 

 consisting in the diploid condition of four pairs, 

 that is to say, pairs which separate in the reduction 

 division so that the gamete contains four single 

 chromosomes, one of each pair. In two of these 

 pairs the chromosomes are elongated and shaped 

 like boomerangs, in the third they are small, round 

 granules, and the fourth pair are the sex-chromo- 

 somes : in the female these last are straight rods, 

 in the male one is straight as in the female, the 

 other is bent. The straight ones are called the 

 X chromosomes, the bent one the Y chromosome. 

 The fertilisations are thus XX which develops into 

 a female fly, and XY which develops into a male. 

 Drosophila therefore is an example of one of the 

 cases described by Wilson. 



Dr. Wilson (loc. ciL) discusses the question of 

 how we are to interpret these facts, in particular, 

 the fact that the X chromosome in fertilisation 

 gives rise to females. He remarks that the X 

 chromosome must be a male-determining factor 

 since in many cases it is the only sex-chromosome 

 in the males, yet its introduction into the egg 



* A Critique of the Theory of Evolution, Princeton University Press 

 and Oxford University Press, 1916. 



