Sagitta bipunctata.
In connection with these insect forms it is of interest to find in the spermatogenesis of Sagitta a body which stains like chromatin and behaves somewhat like the accessory chromosome. It is found in all resting stages of the spermatogonia, closely applied to the nuclear membrane (fig. 226). It divides before each spermatogonial mitosis (fig. 227) and, though not often discernible in the spindle, appears in the next generation. Figure 228 is the last spermatogonial mitosis, and figure 229 shows the element x, and the chromosomes paired at one pole of the spindle. During the various phases of the growth stage (figs. 230-232) the element x is again applied to the nuclear membrane.
In the prophase of the first maturation division this element divides (figs. 233-234), and in metakinesis the two elements are found in various positions with regard to the spindle (figs. 235-237), often as conspicuous as in these figures, but sometimes concealed among the chromosomes. Before the spindle for the second division forms, this element divides again and one of the products goes into each spermatid (figs. 238-241).
As Sagitta is hermaphrodite, there would appear to be no question of sex determination by any special chromatic element. The size of the element x, its evident chromatic nature, its division before each mitosis, and its presence in mitosis and in the spermatids, with the same staining qualities as in the previous rest stages, certainly indicate some important function, either in the whole process of spermatogenesis or in the formation of the sperm-head, of which it finally becomes a part. In Sagitta this element certainly can not be regarded as a specialized spermatogonial chromosome, or as chromatin rejected from the spireme. No such element is present in the ovogenesis of Sagitta (Stevens, '03), nor has any been detected in connection with fertilization. It is certain that none is present in the first segmentation spindle of the egg.