THE DETERMINATION OF SEX 269 



associated with it in the male cells. Thus, in Gelas- 

 tocoris, a hemipteron, the male is represented by the 

 formula 2n + kx + y and the female by 2n -f- &r. 

 Here "n" equals fifteen, so that the male diploid num- 

 ber of chromosomes is thirty-five and the female, 

 thirty-eight. 



Until recently the ^-chromosome has not been known 

 to carry specific genes for bodily characters. Indeed, 

 this chromosome has been generally regarded as merely 

 a degenerate ^-chromosome that has lost its sex genes 

 and most of its other genes as well. That it is 

 essential to the typical development of those species 

 where it is normally present has been proven in the 

 non-disjunction experiments of Bridges. A male 

 Drosophila without the ^-chromosome, for instance, is 

 sterile. 



In many forms it is not unlikely that there is no sex- 

 determining mechanism visible even with the aid of the 

 best microscopes, but, nevertheless, it is probable that 

 x- and ^/-chromosomes exist, and that the ^-chromo- 

 somes are practically equal to the ^-chromosomes in 

 size, differing from them merely in the absence of 

 specific genes. 



B. SEX GENES 



In the female, except in those cases where difference 

 in chromosomal size is present, the ^-chromosomes can- 

 not always be distinguished from ordinary autosomes 

 and it is furthermore known from breeding experiments 

 that they bear many genes for characters having noth- 

 ing to do with sex. 



