STUDIES ON CHROMOSOMES 87 
has no synaptic mate. This condition seems to have arisen in 
more than one way. It is almost certain that in many cases the 
Y-chromosome has disappeared by a process of gradual and pro- 
gressive reduction (as indicated by the graded series observed 
in the Hemiptera (Wilson, ’05b, ’06). In some cases (of which 
Metapodius is an example) the same result may have been pro- 
duced suddenly by a failure of the idiochromosomes to separate 
in the second spermatocyte-division (Wilson, ’09b). <A third pos- 
sibility, first suggested by Stevens (’06), is that the X-element 
may have separated from a YY-pair with which it was originally 
united. This possibility seems to be supported by recent obser- 
vations on Ascaris megalocephala, where the X-chromosome is 
sometimes fused with one of the other pairs, sometimes free 
(Edwards, 710). 
(3) We have as yet no positive knowledge as to how the X- 
member of the XY-pair originally differed, or now differs, from 
the Y, or as to how this difference arose—a definite answer to 
these questions would probably give the solution of the essential 
problem of sex. There are, however, pretty definite grounds for 
the hypothesis that the X-member contains a specific ‘X-chroma- 
tin’ that is not present in the Y-member, and that the X Y-pair 
is heterozygous in this respect. If this be so, the primary sexual 
differentiation is therefore traceable to a condition of plus or 
minus in this pair, accompanied by a corresponding difference 
between the nuclear constitution of the two sexes. (Cf. Wilson, 
10a.) Further, there is also reason for regarding the heterozy- 
gous condition of this pair as due to the presence of the X-chroma- 
tin in one member of a pair which is (or originally was) homozy- 
gous in respect to its other constituents. The latter may be 
called collectively the ‘Y-chromatin’; and we may, accordingly, 
think of the XY-pair as being essentially a YY-pair with one 
member of which the X-chromatin is associated.6 Both the X- 
6 This suggestion is in principle the same as one earlier made by Stevens (’06, 
p. 54) that the Y-chromosome represents ‘‘some character or characters which 
are correlated with the sex-character in some species but not in others,” with one 
member of which the X-chromosome is fused; and that ‘“‘a pair of small chromo- 
somes might be subtracted from the unequal pair, leaving an odd chromosome.”’ 
