Studies on Chromosomes 33 



the modification of sex-production in some cases by external condi- 

 tions, and with the production of both males and females in 

 parthenogenesis (though this may be otherwise explicable); and 

 it might also give the explanation of selective fertilization. 



II. It has not been my intention to advocate the foregoing 

 interpretation, but only to set forth as clearly as possible, the as- 

 sumptions that it involves. It is nevertheless my opinion that the 

 analysis places no insuperable obstacles in its way, and that, 

 however dominance be determined, the Mendelian interpretation 

 may in fact give the true solution of the problem. I have, how- 

 ever, endeavored to seek for a different interpretation that may 

 escape the necessity for assuming a selective fertilization; and 

 although I have to offer nothing more than suggestions, some of 

 which undoubtedly encounter serious difficulties, I shall make 

 them in the hope that they may afford some clue to further inquiry. 

 Some of these suggestions are equally applicable to the Mendelian 

 interpretation considered above, but for the purpose of discussion 

 this interpretation may for the time be laid aside. 



It seems possible that the differential chromosomes may per- 

 form a definite and special function in sex-production without being 

 in themselves specifically male-determining and female-determin- 

 ing or even qualitatively different save in the degree of their special 

 activity (whatever be its nature). This suggestion is given by 

 the fact that the presence of one heterotropic chromosome or 

 large idiochromosome is associated with the production of a 

 male, while if two such chromosomes are present a female is 

 produced. This very obviously suggests that the same kind of 

 activity that produces a male will if reinforced or intensified 

 produce a female; and with this would accord the production 

 of males from unfertilized eggs, and females from fertilized ones, 

 in the case of the bee. In these cases the decisive factor may be 

 a merely quantitative difference of chromatin between the two 

 sexes. But it is obvious that such a difference cannot give the 

 basis for a general explanation, since in Nezara, and presumably 

 in many other organisms, both the number of chromosomes and 

 the quantity of chromatin is the same in both sexes. And yet 



