98 EDMUND B. WILSON 
the experimental results, as Morgan has pointed out. It seems 
probable that all the observed phenomena may be reduced in 
principle to one or the other of these schemes, though many modi- 
fications or complexities of detail probably exist. A possible basis 
for many such modifications seems to be provided by the cyto- 
logical facts already known. 
(2) We might assume that in cases of the first type (e.g., Droso- 
phila) both sex and the characters associated with it are deter- 
mined by the same chromatin; and such a possibility should 
certainly be borne in mind. But in view of the widely different 
nature of the characters already known to exhibit sex-limited hered- 
ity it seems improbable that we can regard them as all alike due 
to the same chromatin. In the light of the conclusions that have 
been indicated in regard to the composition of the X-element, it 
seems more probable that such characters may be determined by 
the various other forms of chromatin (‘Y-chromatin’) associated 
with the X-chromatin. If these constituents be identical with 
those contained in the free Y-chromosome (the synaptic mate of 
X) sex-limited heredity would of course not appear, since the two 
members of the pair would be homozygous in this respect. It 
should make its appearance as a result of the dropping out, or 
other modification, of certain Y-constituents of the X-element, and 
such a mutation might arise in either sex. 
We may perceive here the possibility of understanding many 
different kinds of sex-limited heredity, perhaps of complex types 
that have not yet been made known. Such a possibility is sug- 
gested, for example, by the remarkable relation discovered by 
McClung (05) in Mermiria (fig. 6f—h, fig. 7 in diagram), where 
the X-chromosome is In the first spermatocyte-division attached 
at one end to a linear chain of four other elements to form a 
pentad complex, to which may be given the formula XA. ABB. 
This so divides as to separate XA from ABB. The interpretation 
to be placed upon this is a puzzling question under any view, and 
apparently must await more extended studies on both sexes, per- 
haps also on other forms, before it can be fully cleared up. Even 
here the possibility exists, I think, that the entire complex may 
have arisen by the differentiation of a single original X Y-pair; 
