of a
segmentation spindle (fig. 225) of the parthenogenetic egg, where 10
chromosomes are present, 2 of each of the sizes found in the sexual germ
cells.

So far as an accessory chromosome or any other visible evidence of a sex
determinant are concerned, the results are entirely negative. The
conditions shown do, however, support Mendel's conception of the "purity
of the germ-cells," and also afford evidence in favor of Boveri's theory
of the individuality of the chromosomes.

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
se