the second
order, persists during the rest stage, appears in the second mitosis as
a dyad and then divides, going into one-half of the spermatids. The
spermatids, however, as in _Stenopelmatus_, all have the same
appearance: each has in the center--not against the nuclear membrane--a
small element that stains like chromatin. Occasionally a mass of
chromatin is found outside the nucleus, but this is not constant enough
to support the contention of Moore and Robinson ('05) that the
"nucleolus" of the related form, _Periplaneta americana_, is fragmented
and cast out into the cytoplasm. The spermatids all appear to develop
equally well for some time, but as they approach maturity a varying
proportion of them become degenerate. This can not, however, be due to
absence of the accessory chromosome, as Miss Wallace supposes, in the
spider; for in some follicles no degenerate spermatozoa are found, and
in others more than half may be degenerate. All attempts to study
fertilization stages of the egg have so far failed, and the chromosomes
in the female somatic cells have not proved favorable for counting.
Twenty-three have been counted in several cases, but there was always
some chance of error. If 23 is the somatic number in both sexes, it must
be maintained by union of sex-cells containing 11 and 12 chromosomes,
respectively, the same unequal number occurring in the maturated eggs as
in the sperm. Under such conditions it is difficult to see how the odd
chromatin element of the spermatozoa can determine sex.

The brief description of the chromatin element _x_ in _Sagitta_,
introduced here because it behaves like the accessory chromosome in many
particulars, serves as an example of the occurrence of such an element
in the spermatogenesis of a hermaphrodite form, where it can not
possibly be conceived of as a sex determinant. In _Sagitta_ it is known
to be confined to the male germ-cells. No such element occurs in the
ovogenesis, in the sperm nucleus in the egg, or in the first
segmentation spindle. Its function must, therefore, be confined to the
process of spermatogenesis.

From the standpoint of sex determination, we have in _Tenebrio molitor_
the most interesting of the forms considered in this paper. In both
somatic and germ cells of the two sexes there is a difference not in the
number of chromatin elements, but in the size of one, which is very
small in the male and of the same size as the other 19 in the female.
The egg