156 SEX-DETERMINATION [CH. 



follows that half the spermatids contain it and half are 

 without it, so that the resulting spermatozoa are of two 

 kinds which are produced in equal numbers (PI. XVII, 



i-5). 



After these phenomena had been observed in the sper- 



matogenesis of several insects, a comparison was naturally 

 made of the oogenesis of the saie species, and it was then 

 discovered that in each case the female contained two 

 similar chromosomes corresponding with the heterotropic or 

 ^T-chromosome of the male. These usually differ from the 

 .AT-chromosome of the male in not forming a chromatin 

 nucleolus in the spireme stage of the oocyte; they behave 

 at this stage like the other chromosomes and unite to form 

 a bivalent element, the two members of which separate in 

 the first maturation division of the egg and divide equation- 

 ally in the second, with the result that all the mature eggs 

 contain a single ^-chromosome. 



From these facts it was natural to infer that the X- 

 chromosome acts as a determiner of sex, for since all the 

 eggs before fertilisation contain one such body, and half 

 the spermatozoa contain it while half do not, it follows that 

 half the fertilised eggs will contain one X and the other half 

 two, an expectation which has been verified by the exam- 

 ination of the mitotic figures in the segmentation of the 

 eggs. It is known, however, that adult males of these species 

 contain one .XT-chromosome while females contain two, and 

 it seems natural to infer that the spermatozoa which con- 

 tain an -XT-chromosome cause the eggs which they fertilise 

 -to v become females, while those which lack the X are male- 

 determining. Some further consideration of this conclusion 

 will be given at a later stage. 



An obvious objection to this hypothesis of sex-deter- 

 mination was at first a difficulty. The known cases in which 



