simple
factors. By following up the evidence as to segregation, indications
have been obtained which can only be interpreted as meaning that when
many factors are being simultaneously redistributed among the
germ-cells, certain of them exert what must be described as a
repulsion upon other factors. We cannot surmise whither this discovery
may lead.

In the new light all the old problems wear a fresh aspect. Upon the
question of the nature of Sex, for example, the bearing of Mendelian
evidence is close. Elsewhere I have shown that from several sets of
parallel experiments the conclusion is almost forced upon us that, in
the types investigated, of the two sexes the female is to be regarded
as heterozygous in sex, containing one unpaired dominant element,
while the male is similarly homozygous in the absence of that
element.[68] It is not a little remarkable that on this point--which
is the only one where observations of the nuclear processes of
gameto-genesis have yet been brought into relation with the visible
characteristics of the organisms themselves--there should be
diametrical opposition between the results of breeding experiments and
those derived from cytology.

Those who have followed the researches of the American school will be
aware that, after it had been found in certain insects that the
spermatozoa were of two kinds according as they contained or did not
contain the accessory chromosome, E. B. Wilson succeeded in proving
that the sperms possessing this accessory body were destined to form
_females_ on fertilisation, while sperms without it form males, the
eggs being apparently indifferent. Perhaps the most striking of all
this series of observations is that lately made by T. H. Morgan,[69]
since confirmed by von Baehr, that in a Phylloxeran two kinds of
spermatids are formed, respectively with and without an accessory (in
this case, _double_) chromosome. Of these, only those possessing the
accessory body become functional spermatozoa, the others degenerating.
We have thus an elucidation of the puzzling fact that in these forms
fertilisation results in the formation of _females_ only. How the
males are formed--for of course males are eventually produced by the
parthenogenetic females--we do not know.

If the accessory body is really to be regarded as bearing the factor
for femaleness, then in Mendelian terms female is DD and male is DR.
The eggs are indifferent and the spermatozoa are each male, _or_
f