CHROMOSOMES, HEREDITY AND SEX. 
511 
in different cases, and aiiotlier explanation, equally concordant 
with the known facts of chromosome behaviour, is available. 
It has been shown by several observers (66, 47) that the 
sex-chromosome is not infrequently compound, and in one 
case (49) the two parts are constantly separate, though they 
are reported always to go to the same pole of the spindle. 
If now one portion of the “ sex-chromosome ” bears the sex- 
factor, the other portion the sex-limited factors, failure of 
sex-limited transmission will ai-ise whenever the components 
of the chromosome become separate, and go to different poles. 
The frequency with which this happens may be expected to 
vary in different cases. This suggestion, due originally to 
AVilson (66), really differs from Morgan’s only in the sup- 
position that the sex-chromosome is commonly coupled with a 
chromosome which bears the sex-limited factors, instead of 
assuming that these factors are borne in the sex-chromosome 
itself. The exceptions to the normal sex-limited transmission 
make this latter assumption untenable.^ 
The facts of sex-limited transmission thus support the hypo- 
thesis that both ordinary Mendelian factors and the sex- 
determining factor or factors are borne by chromosomes, 
although the details of their relations are still very far from 
being clear.- That there is some intimate relation, how- 
ever, between a chromosome and the transmission of sex- 
limited characters is almost certain from the fact that sex- 
' Since this was written Bridges (12a) has suggested a hypothesis 
to avoid this difficulty. He assumes that exceptions are caused by 
“ non-disjunction ” of the sex-chromosomes, so that both go into one 
germ-cell. This explanation, if substantiated, would account for most 
of the recorded exceptions, but not that of the tortoiseshell male cat, 
which contains both the colour-factors characteristic of the female, but 
must be supposed to have only one sex-chromosome. 
- The recently published work of Miss K. Foot and Miss E. C. Strobell 
(‘ Biol. Bull.,’ xxiv, 1913, p. 187) cannot be used as an argument against 
this proposition. They have shown (as was previously known in birds and 
moths) that a secondary sexual character in Hemiptera can Idc trans- 
mitted through the sex which does not show it, but the character was 
not sex-limited in transmission ; their results, therefore, have no bearing 
in the present discussion. 
