tin element, which corresponds fairly
well to the large or to the small component of the unsymmetrical pair,
separated in the first mitosis and divided in the second. The clear
portion of the nucleus containing this isolated element is at first
turned toward the spindle-remains (fig. 198), but before the tail
appears either the whole nucleus or its contents have rotated 180 deg.
(fig. 199). Various stages in the development of the spermatid are seen
in figures 200 to 203. The clear region and the isolated element finally
disappear (fig. 202 _b_), and the chromatin breaks up into coarser and
then into finer granules within the sperm-head. In the later stages the
centrosome is clearly seen at the base of the head (fig. 203).

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 t