The egg which receives a spermatozoön without an X chromosome has after fertilization 12+X chromosomes and develops into a male; while the egg into which a spermatozoön with an X chromosome enters gives rise to a female. Since all the body cells arise from the fertilized egg by nuclear division and the chromosomes remain constant in number in all cells, the consequence is that all the cells of a female _Protenor_ have two X chromosomes; while all the cells of a male _Protenor_ have only one X chromosome. [Illustration: FIG. 39] The chromosome situation in _Protenor_ is a somewhat extreme case, inasmuch as one X chromosome is entirely lacking in the male. In other forms of Hemiptera, _e. g._, _Lygæus_, there are also two types of spermatozoa appearing in equal numbers differing in regard to the X chromosome, but here it is only a difference in size; one-half of the spermatozoa having a large X chromosome, the other half instead a smaller chromosome. Calling this latter the Y chromosome, the sex determination in this form is as follows: leaving aside the chromosomes which are equal in both egg and spermatozoön we may say that there is one type of egg containing one large X chromosome; there are two types of spermatozoa in equal numbers, one possessing a large X chromosome, the other possessing a small Y chromosome. Wilson showed by a study of the chromosomes in males and females that when one of the spermatozoa containing a large X chromosome enters the egg, the egg will develop into a female; while when one of the spermatozoa containing a small Y chromosome enters it will give rise to a male. Leaving aside the common chromosomes of both sexes, a fertilized egg containing XX gives rise to a female, while one containing XY gives rise to a male. There is in this case as in that of _Protenor_ a preponderance of chromosome material in the female, but this quantitative difference is not essential for the determination of sex, since in some species the Y chromosome may be as large as the X chromosome. The main fact is that the female cells have the chromatin composition XX, the male cells the composition XY, where Y is apparently qualitatively different and often, but not necessarily, smaller than X, or entirely lacking. It may be mentioned in passing that indirect evidence exists indicating that in man there are also two kinds of spermatozoa and one kind of egg, and that sex depends on whether a male determining or a female determining spermatozoön enters the egg.