1 88 Cytological Evidence as to Sex [ch. 



In his discussion of these curious numbers Castle calls 

 attention to. the fact that the results of DR x RR are very 

 nearly 2D^ : iR$ : \D% : 2R%, and developing his view that 

 each sex is heterozygous in sex, he suggests that the 

 gametes of DR females and males may bear the sex- 

 characters and the colour-characters coupled in this way, 

 forming a series 2 + i + i + 2. Assuming also that in 

 fertilisation union can only take place between gametes of 

 opposite sex, the F^ numbers would be ZD^ : \R^\6D% 137??, 

 a series which, as he points out, fits the total of the observed 

 numbers extraordinarily well. 



Recognizing the great interest of the case I fe^l, never- 

 theless, that so long as it stands entirely alone, we are 

 justified in treating it as of somewhat doubtful significance. 

 We know from divers sources that the sex-ratios of the 

 Lepidoptera are liable to astonishing fluctuations. Very 

 large families consisting of all, or nearly all, females, or 

 males, as the case may be, having been not rarely witnessed, 

 and until experiments with tau and lugens are repeated with 

 a full understanding of the importance that may attach to 

 them, we may postpone positive conclusions. 



The Cytological Evidence. 



We have now to mention a group of facts which, though 

 agreeing with the general conclusion that one sex is hetero- 

 zygous and the other homozygous, suggests that in the 

 types concerned, the roles are reversed. 



From the cytological side a remarkable advance in the 

 problem of sex has been made in the discovery of the 

 accessory chromosome in the spermatogenesis of certain 

 Insects. McClung, studying this structure, originally ob- 

 served by Henking, was the first to insist on its importance. 

 He showed that in certain Insects half the sperms have 

 it and half are without it. This fact led him to make 

 the natural suggestion that the structure might be con- 

 cerned in the differentiation of sex. This suggestion has 

 been shown by E. B. Wilson to be correct, but the accessory 

 body proves to be the peculiarity of the sperms which are 

 destined to form females, not of those which will form 

 males, as had been previously supposed. The evidence for 

 this is the fact that while the number of chromosomes in 

 the male cells of the species concerned is either n, or n— i. 



