permatogenesis exists in at least 3
families, where an odd chromosome like that in the Orthoptera occurs in
place of the unequal pair. In all of these insects the spermatozoa are
distinctly dimorphic, forming two equal classes, one of which either
contains one smaller chromosome or lacks one chromosome.

The most difficult part of the work has been the determination of the
somatic number of chromosomes in the male and female. In some cases
suitable material has been lacking; in others, though material was
abundant, no metaphases could be found in which the chromosomes were
sufficiently separated to be counted with certainty. In three species
(in addition to _Tenebrio molitor_) where the unequal pair is present,
the female somatic cells have been shown to contain the same number of
chromosomes as the spermatogonia, but an equal pair in place of the
unequal pair of the male. In two new cases the male somatic number and
size have been shown to be the same as in the spermatogonia. In one of
the Elateridae, where the spermatogonial number is 19, the female somatic
number is 20, and in _Aphrophora_ the numbers in male and female cells
are respectively 23 and 24. No exception has been found to the rule
established by previous work on the Coleoptera (Stevens, '05) and on the
Hemiptera (Wilson, '05 and '06), that (1) in cases where an unequal pair
is present in the male germ cells, it is also present in the male
somatic cells, but is replaced in the female by an equal pair, each
component being equal in volume to the larger member of the unequal
pair, and (2) in cases where an odd chromosome occurs in the male, a
pair of equal size are found in the female. It is therefore evident that
an egg fertilized by a spermatozoon (1) containing the small member of
an unequal pair or (2) lacking one chromosome, must develop into a male,
while an egg fertilized by a spermatozoon containing the larger element
of an unequal pair of heterochromosomes or the odd chromosome must
produce a female.

Whether these heterochromosomes are to be regarded as sex chromosomes in
the sense that they both represent sex characters and determine sex, one
can not decide without further evidence.

Comparison of the two types in Coleoptera, especially where, as in the
Carabidae, both occur in one family, has suggested to me that here it is
possible that the small chromosome represents not a degenerate female
sex chromosome, as suggested by Wilson, but some cha