chromosome. The fertilisations are thus XX which develops into a female
fly, and XY which develops into a male. Drosophila therefore is an example
of one of the cases described by Wilson.

Dr. Wilson (_loc. cit._) discusses the question of how we are to interpret
these facts, in particular, the fact that the X chromosome in
fertilisation gives rise to females. He remarks that the X chromosome must
be a male-determining factor since in many cases it is the only
sex-chromosome in the males, yet its introduction into the egg establishes
the _female_ condition. This is the same difficulty which I pointed out
above in connection with the Mendelian theory that the female was
heterozygous and the male homozygous for sex. Dr. Wilson points out that
in the bee, where fertilised eggs develop into females and unfertilised
into males, we should have to assume that the _X_ chromosome in the female
gamete is a female determiner which meets a recessive male determiner in
the _X_ chromosomes of the sperm. When reduction occurs, the _X_[female]
must be eliminated since the reduced egg develops always into a male. But
on fertilisation, since the fertilised egg develops into a female, a
dominant _X_[female] must come from the sperm, so that our first
assumption contradicts itself.

Dr. Wilson, T. H. Morgan, and Richard Hartwig have therefore suggested
that the sex-difference as regards gametes is not a qualitative but a
quantitative one. In certain cases there is no evident quantitative
difference of chromatin as a whole, but there may in all cases be a
difference in the quantity of special sex-chromatin contained in the _X_
element. The theory put forward by Wilson then is that a single _X_
element means _per se_ the male condition, while the addition of a second
element of the same kind produces the female condition. Such a theory
might apply even to cases where no sex-chromosomes can be distinguished by