quadruple, while the other, say _Y_, is single. In all these cases there
are two _X_ chromosomes in the oocytes (and somatic cells) of the female,
and after reduction the female gametes or unfertilised ova are all alike,
having a single _X_ chromosome or group. On fertilisation half the zygotes
have _XX_ and half _XY_, whether _Y_ is absence of a sex-chromosome,
or one of the other _Y_ forms above mentioned. The sex is thus determined
by the male gamete, the _X_ chromosome united with that of the female
gamete producing female individuals, while the _Y_ united with _X_
produces male individuals.

Professor T. H. Morgan has made numerous observations and experiments on a
single culture of the fruit-fly, _Drosophila ampelophila_, bred in bottles
in the laboratory for five or six years. He has not only studied the
chromosomes in the gametes of this fly, and made Mendelian crosses with
it, but has obtained numerous mutations, so that his work is a very
important contribution to the mutation doctrine. Drosophila in the hands
of Professor Morgan and his students and colleagues has thus become as
classical a type as Oenothera in those of the botanical mutationists.
Different branches of Morgan's work are discussed elsewhere in this
volume, but here we are concerned only with its bearing on the question of
the determination of sex. He describes [Footnote: _A Critique of the
Theory of Evolution_. Princeton University Press and Oxford University
Press, 1916.] the chromosomes of Drosophila as consisting in the diploid
condition of four pairs, that is to say, pairs which separate in the
reduction division so that the gamete contains four single chromosomes,
one of each pair. In two of these pairs the chromosomes are elongated and
shaped like boomerangs, in the third they are small, round granules, and
the fourth pair are the sex-chromosomes: in the female these last are
straight rods, in the male one is straight as in the female, the other is
bent. The straight ones are called the X chromosomes, the bent one the Y