STUDIES ON CHROMOSOMES 99 



but this question is clearly not yet ready for discussion. How- 

 ever such associations have arisen, the result is equally appli- 

 cable to the explanation of sex-limited heredity. 



(d) Secondary sexual characters. Castle ('09) has offered the 

 interesting suggestion that the free Y-chromosome may be re- 

 sponsible for the determination of secondary sexual characters 

 in the male. Though I have criticized this view ('09c) I now 

 believe it may be true for certain cases. It is obviously excluded 

 when the Y-chromosome is missing; and since nearly related 

 species in Metapodius even different individuals of the same spe- 

 cies show the same or similar secondary male characters whether 

 this chromosome be present or absent, it seems probable that 

 these characters are in general determined in some other way. 

 But if, as I have suggested, sex-limited heredity may arise through 

 a modification of the Y-constituents of the X-element, it follows 

 that the YY-pair thereby becomes heterozygous. In such case, 

 the free Y-chromosome, being confined to the male line, should 

 continue to represent characters that are no longer present in 

 the female, and hence would be indistinguishable from secondary 

 male characters otherwise determined. It has further become 

 evident (as is indicated below) that the chromosome-groups are 

 so plastic that their specific composition may vary widely from 

 species to species. It may very well be, therefore, that Castle's 

 suggestion may apply to some forms. 



6. Modes in which the chromosome-number may change 



The constant and characteristic duality of the 'd-chromosome' 

 in the second division suggests a series of questions regarding the 

 mode in which the chromosome-number may change that have 

 an important bearing on those already considered. The appear- 

 ance of this chromosome must suggest to any observer that it is 

 a compound body, consisting of two closely united components 

 that are invariably associated in a definite way; but it is especially 

 noteworthy that its duality does not certainly appear before the 

 last division. This case must be added to the steadily increasing 

 evidence that chromosomes which appear single and homoge- 



