98 EDMUNP 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. Qf-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 XY-pair; 



