THE DETERMINATION OF SEX 585 



cell. It is, of course, possible to assume that the inequality 

 is itself determined by the asymmetrical distribution of the 

 X-chromosome ; but on the whole it seems more natural to 

 conclude that the X-pole is already predestined before the 

 X-element passes into it. Again, as Morgan discovered in 

 Phylloxera, the males, produced parthenogenetically, possess but 

 one X-element instead of two (thus conforming to the general 

 rule), one of the X-elements being eliminated in the maturation 

 of the male-producing egg ; but this egg is already distinguish- 

 able as male-producing, by its smaller size, before the elimination 

 takes place. It thus becomes rather a nice question whether the 

 egg should be regarded as actually male before the characteristic 

 male chromosome-combination has been established. We may, 

 I think, reasonably take the epigenetic view that sex is first 

 really established or determined when all the essential sex- 

 producing factors have been brought together in the egg. The 

 chromosome-combination may be taken as the culmination of 

 this process, and hence as a convenient point of departure. 

 The following discussion is based upon the assumption (there 

 is no complete proof of the fact) that the X-chromosomes are 

 one of the essential factors ; and if the foregoing qualifications 

 are not lost sight of, we shall not, I think, be seriously misled 

 by the further assumption that they are the decisive factor. 

 With this explanation let us examine some of the possibilities 

 that present themselves. 1 



Our first impression is that the X-chromosomes must be 

 the bearers of specific and opposing factors (such as different 

 chemical substances) that are per se male-determining or female- 

 determining ; but this view speedily involves us in a maze of 

 difficulties, of which only a few will here be considered. For 

 instance, under this view the single X-chromosome of the male 

 must be a male-determining factor (since in the males of many 

 species it is the only sex-chromosome present); yet its intro- 

 duction into the egg establishes the female condition. We are 

 therefore driven into the following additional assumptions : (1) 



1 The discussion of these complex possibilities may be simplified if for the 

 present we leave the Y-element out of consideration. I do not mean to imply that 

 this element is of no importance. On the contrary, until its significance has been 

 made clear, no statement of results can be regarded as more than provisional. 

 Since, however, the Y-element may be either present or absent, even in different 

 individuals of the same species (Metapodius), we must perforce for the present lay 

 the question on the table. 



