In order to determine, if possible, the value of the unsymmetrical pair of chromatin elements, very young ovaries and ovaries with egg-tubes were sectioned and the chromosomes counted in the dividing cells of the egg-follicle (Female somatic cells), and in dividing oogonia. In both cases 20 large chromosomes were found. Figure 207 is the equatorial plate from a female somatic cell of a young egg-follicle. Figure 208 _a_ and _b_ shows two sections of an oogonium in the prophase of mitosis. In order to determine the number and character of the chromosomes in the male somatic cells, several male pupae were sectioned. As in the spermatogonia, 19 large chromosomes and 1 small one were found. Figure 204 shows the equatorial plate of a dividing male somatic cell, and figures 205 to 206 are daughter plates from a similar cell. (Three large chromosomes of the plate shown in figure 206 are in another section.) From these facts it appears that the egg-pronucleus must in all cases contain 10 large chromosomes, while the spermatozoon in fertilization brings into the egg either 10 large ones or 9 large ones and 1 small one. Since the somatic cells of the female contain 20 large chromosomes, while those of the male contain 19 large ones and 1 small one, this seems to be a clear case of sex-determination, not by an accessory chromosome, but by a definite difference in the character of the elements of one pair of chromosomes of the spermatocytes of the first order, the spermatozoa which contain the small chromosome determining the male sex, while those that contain 10 chromosomes of equal size determine the female sex. This result suggests that there may be in many cases some intrinsic difference affecting sex, in the character of the chromatin of one-half of the spermatozoa, though it may not usually be indicated by such an external difference in form or size of the chromosomes as in _Tenebrio_. It is important that related forms should be studied in order to ascertain whether the same chromatic conditions prevail in other species of this genus or possibly in the Coleoptera in general.[A] [Footnote A: Prof. E. B. Wilson has recently found a similar dimorphism in the spermatozoa of _Lygaeus_ and other of the _Hemiptera heteroptera_.] Aphis oenotherae. The spermatogenesis of _Aphis_ has been fully described in another paper and will merely be briefly summarized here for the purpose of comparison with other forms.