with 2_N_ chromosomes, and male-producing eggs with 2_N_-1 or 2_N_-2
chromosomes. There is also evidence that in some cases, _e.g._ the
sea-urchin, the female is heterozygous, forming gametes, some with _N_ and
some with _N_+ chromosomes, while the male gametes are all _N_.
Fertilisation then produces male-producing eggs with 2_N_ chromosomes,
female-producing with 2_N_+.

Such is the summary given by Castle in 1912. [Footnote: _Heredity and
Eugenics_, by Castle and Others. University of Chicago Press, 1912.] It
will be seen that he treats the differences as purely quantitative, mere
differences in the number of the chromosomes. Professor E. B. Wilson,
however, who had contributed largely by his own researches to our
knowledge of sex from the cytological point of view, had already
published, in 1910, [Footnote: '_The Determination of Sex_,' _Science
Progress_, April 1910.] a very instructive _resume_ of the facts observed
up to that time. The important fact which is generally true for insects,
according to Wilson, is that there is a special chromosome or chromosomes
which can be distinguished from the others, and which is or are related to
sex differentiation. This chromosome, to speak of it for convenience in
the singular, has been variously named by different investigators. Wilson
called it the 'X chromosome,' McCluny the 'accessory chromosome,'
Montgomery the 'hetero-chromosome,' while the names 'heterotropic
chromosome' and idiochromosome have also been used. For the purpose of the
present discussion we may conveniently name it the sex-chromosome. It is
often distinguished by its larger size and different shape. Wilson
describes the following different cases:--

(1) The sex-chromosome in the male gametocytes is single and fails to
divide with the others, but passes undivided to one pole. This may occur
in the first reduction division (Orthoptera, Coleoptera, Diptera) or in