while an ovum containing one X sex-chromosome, or two different, XY,
chromosomes, at the next reduction division gives rise to spermatozoa. The
X sex-chromosome is not in itself either female or male, since, as we have
seen, either ovum or spermatozoon may contain a single X chromosome. The
ovum then with one X chromosome or one X and one Y changes its sex at the
next reduction division and becomes male. In parthenogenetic ova this
happens without conjugation with a spermatozoon at all: in other cases,
since the zygote is compounded of spermatozoon and ovum, we can only say
that in the XX zygote, the ovum developing only ova, the female is
dominant, in the X or XY zygote developing only spermatozoa the male is
dominant. Hermaphrodite animals, as has been pointed out by Correns and
Wilson, cannot be brought under this scheme at all. In the earthworms, for
instance, we have, in every individual developed from a zygote, ova and
spermatozoa developing in different gonads in different parts of the body.
The differentiation here, therefore, must occur in some cell-division
preceding the reduction divisions. Every zygote must have the same
composition, and yet give rise to two sexes in the same individual.

Further light on the sex problem, as in many other problems in biology,
can only be obtained by more knowledge of the physical and chemical
processes which take place in the chromosomes and in the relations of
these structures to the rest of the cell. The recent advances in cytology,
remarkable as they are, consist almost entirely of observations of
microscopic structure. They may be said to reveal the statics of the cell
rather than its dynamics. Cytology is in fact a branch of anatomy, and in
the anatomy of the cell we have made some progress, but our knowledge of
the physiology of the cell is still infinitesimal. The nucleus, and
especially the chromosomes, are supposed in some unknown way to influence
or govern the metabolism of the cytoplasm. From this point of view the
hypothesis mentioned above that the sex-difference in the gametes is not