racter or characters
which are correlated with the sex character in some species and not in
others. Assuming this to be the case, a pair of small chromosomes might
be subtracted from the unequal pair, leaving an odd chromosome. The two
types would then be reduced to one. It may be possible to determine the
validity of this suggestion for particular cases by observation or
experiment.

Since the first of this series of papers was published, there have
appeared three important papers by Prof. E. B. Wilson, bearing on the
problem of sex determination in insects. These papers are based on a
study of many species of the Hemiptera heteroptera. These insects fall
into two classes--one in which a pair of "idiochromosomes," usually of
different size, remain separate and divide quantitatively in the first
spermatocyte, conjugate and then separate in the second maturation
mitosis; and another class in which an odd chromosome--the
"heterotropic" chromosome--divides in one of the maturation mitoses, but
not in the other. Wilson regards the odd chromosome as the equivalent of
the larger of the "idiochromosomes," its smaller mate having
disappeared. In the somatic cells of the former class he finds in the
male the unequal pair, in the female an equal pair, the smaller
chromosome being replaced by an equivalent of the larger
"idiochromosome." In the latter class the male somatic cells contain the
odd number, the female somatic cells and oogonia an even number, the
homologue of the odd chromosome of the male being present and giving to
the female one more chromosome than are found in the male.

In his latest paper Wilson ('06) makes a variety of suggestions as to
sex determination. He shows that if the "idiochromosomes" and the
heterotropic chromosome be regarded as sex chromosomes in the double
sense that they both bear sex characters and determine sex, the
following scheme accounts for the observed facts in all cases where an
unequal pair or an odd heterochromosome have been found:

Sperm. Egg.
{Large [Male] "idiochromosome"}
I. {or } + Large [Female] sex chromosome = a [Female]
{Odd chromosome. }

II. {Small [Female] "idiochromosome"}
{or } + Large [Male] sex chromosome = a [Male]
{No sex chromosome }

Here we know that such a combination of gametes must occur to give the
observed results, b