In many insects and other Arthropods which are not parthenogenetic the male has been found to possess fewer chromosomes than the female. The female forms, as in the above cases of parthenogenesis, only gametes of one kind each with _N_ chromosomes, but the male forms gametes of two sorts, one with N chromosomes, the other with _N_-l or _N_-2 chromosomes. On fertilisation two kinds of zygotes are formed, female-producing eggs 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 _résumé_ 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:--