260 EdmundB. Wilson: 
forms in equal numbers. Further, it now seems certain that in 
sex-limited heredity — such as appears in the heredity of the 
color-pattern in Abraxas (Doncaster and Raynor), of color- 
blindness in man, or of eye-color in Drosophila (Morgan) — 
these somatie characters are linked with a sex-determing factor 
in respect to which one sex is homozygous, the other heterozygous. 
The facts also show that in some of these cases the heterozygous 
sex is the female (Abraxas, Plymouth Rock fowls), in others the 
male (Bryonia, Drosophila). This is precisely parallel to what is 
revealed by the cytological evidence; for what I have called the 
"homogametic‘ and the ”digametic“ sexes obviously correspond 
respectively to the homozygous and the heterozygous conditions. 
It is possible to maintain, as some writers have done, that 
the sex-chromosomes are not a determining cause but only an 
accompaniment of sex. I do not myself consider them as sex- 
determinants in any exclusive sense. I do regard them as one 
link — probably an essential one — in a chain of factors by 
which sex is determined and inherited; and since they are the 
most accessible and obvious of these factors, we may for purposes 
(1899) described and figurel this chromosome as a compact body in the 
resting "spermatogonial“ nuclei; but the cells thus designated were no doubt 
early spermatocytes, as he has himself since recognized. 
My first and second Studies“ (1905), supplemented by the following 
three (1906, 1909) established the following facts in the Hemiptera. 
1. That the single large chromatic ”nucleolus“ of the growth-period 
in species having an unpaired X-chromosome does not arise from the 
m-chromosomes, but from a single larger spermatogonial chromosome. It is 
therefore univalent, as in Orthoptera. 
2. That this chromosome-nucleolus is identical with the "heterotropie“ 
chromosome (X-chromosome) of the spermatocyte-divisions. 
3. That the m-chromosomes are of quite different nature from Mont- 
gomerys unequal ”chromatin-nucleoli“ or "unequal diplosomes“, since they 
differ widely in behavior from the latter, and may co-exist with them in 
the same species (Metapodius). 
4. That the unequal ”chromatin-nucleoli“ are an XY pair, of which 
the larger member is the homologue of the odd or unpaired X-chromosome. 
5. That the sexes are identical in respect to all the chromosomes 
(ineluding the m-chromosomes) excepting X and Y, which show the charac- 
teristic differences that have heen reviewed in the foregoing text. 
As regards the "resting‘ spermatogonial nuclei, there is no a priori 
reason why the nucleoli that are often seen in them should not, in some 
cases, be the sex-chromosomes. Thus far, however, I have been unable to 
