The Sex Chromosomes. 267 
of ”trophochromatin‘“; and in a more recent discussion (1910) 
he develops the idea that sex-production may be determined by 
the greater or less quantity of a substance that affects the total 
energy-production of the organism. He thus endeavors to reconcile 
the eytologieal discoveries with experimental results that have 
seemed to show the sex-determining effects of nutritio nor other 
external agencies. 
I am in sympathy with such a conception, but have also 
earlier pointed out the obvious difficulty (if it raally is a diffi- 
culty) that the characters of the female can not be regarded as 
merely an exaggeration or intensification- of those of the male. 
For this reason it seems to me premature to go far beyond the 
facts actually seen. It is for the present safer to conclude only 
that development is turned into either the male or the female 
direction by some speeifie but unknown action induced by the 
presence in the zygote of more or less of the specific X-material. 
Obviously, in. cases where the female is the digametic sex we 
can not assume the female formula to be XX. We may never- 
theless still hold to the quantitative prineiple, asceribing to the 
female the presence of a specific X-material that is either deficient 
or wanting in the male. Castle has suggested that in these cases 
the female may contain one X and the male none; but it remains 
to be seen whether this correctly expresses the actual facts. 
Essentially, the quantitative interpretation accords with the 
"presence and absence‘“ theory of Mendelian heredity, as Castle 
first pointed out (1909). The principal difficulty in its way, as 
Correns has urged (1907) is offered by the phenomena of 
hermaphroditism — a difficulty of the same nature as is presented 
by the pied or spotted types of pigmentation in many organisms. 
This diffieulty is, I believe, by no means insuperable; but the 
question is clearly not yet ready for discussion. 
The limits of this review would not permit an adequate 
discussion of the eytological results in their relation to the effeets 
of external conditions on sex-production. I have always held 
open the possibility expressed in my third Study“ that "the 
sex of the fertilized eggs may be only a matter of greater or 
less pre-disposition, and not an immutable pre-determination“. 
Nevertheless, it can hardly be denied that both the cytological 
facts and the main weight of the experimental results are now 
