The Sex Chromosomes. 263 
pretation is more likely to be correct. In my first outline of this 
interpretation (Study III) I endeavored to account for both the 
X- and the Y-chromosomes by assuming them to differ in degree 
of activity, the X-chromosome being supposed to consist of a less 
active chromatin than the X. 
”Under this assumption the facts might receive a general 
formulation in the statement that the association of two more 
active chromosomes of this class produces a female, while the 
association of a more active and a less active one (or the absence 
of the latter) produces a male. Reduction of the less active 
member to form a small idiochromosome would introduce a 
quantitative difference of chromatin as well as qualitative one.') 
Its complete disappearance in the male, leaving only the active 
member as the heterotropie chromosome, would reduce the 
difference to a merely quantitative one‘ (1906). 
This differs from all earlier quantitative conceptions of sex- 
determination (such as those of Morgan or of R. Hertwig) in 
ascribing the relation to the specific chromatin of a particular 
pair of chromosomes. It was adopted by Morgan in his Experi- 
mental Zoology (1907), but with some limitation; for he 
restricted the quantitative relation to chromatin mass, ascribing 
the result of fertilization to ”the greater amount of chromatin 
brought into the egg with the accessory‘‘ (p. 405). This assumption 
was included in my own hypothesis, as the foregoing eitation 
shows; but I went a step farther in the attempt to explain also 
cases in which no difference of total mass appears between X 
and Y, or between the sexes.) Morgan argued that in such 
cases the quantitative factor disappears. It is clear, however, 
that this argument does not hold if any difference of substance 
exists between X and Y. My own hypothesis postulated such a 
difference, and one that is still quantitative in a physiological 
sense; but even if the difference be qualitative (as I now incline 
to think it is) the quantitative mass-relation of the X-chromatin 
evidently still holds true. 
') The word "”qualitative‘‘ was here not well chosen. However, both 
this passage in itself and the preceding context make it clear that it was 
meant to indicate only a difference of relative activity. The result is the 
same if an actual qualitative difference between X and Y be assumed. 
°) Cf. the foregoing remarks at p. 6. 
