ut we are not certain that we have a right to
attribute the sex characters to these particular chromosomes or in fact
to any chromosomes. It seems, however, a reasonable assumption in
accordance with the observed conditions. The scheme also assumes either
selective fertilization or, what amounts to the same thing, infertility
of gametic unions where like sex chromosomes are present. It also
assumes that the large female sex chromosome is dominant in the presence
of the male sex chromosome, and that the male sex chromosome is dominant
in the presence of the small female sex chromosome. Or, it might rather
be said that these are not really assumptions, but inferences as to what
must be true if the heterochromosomes are sex chromosomes. This theory
of sex determination brings the facts observed in regard to the
heterochromosomes under Castle's modification of Mendel's Law of
Heredity ('99).

The question of dominance is a difficult one, especially in
parthenogenetic eggs and eggs which are distinctly male or female before
fertilization. It may be possible that the sex character of the egg
after maturation is always dominant in the fertilized egg, as appears to
be the case in these insects (see scheme). Conditions external to the
chromosomes may determine in certain cases, such as Dinophilus, which
sex character shall dominate in the growing oocyte, and maturation occur
accordingly. It is evident that this reasoning would lead to the
conclusion that sex is or may be determined in the egg before
fertilization, and that selective fertilization, or infertility of
gametic unions containing like sex characters, has to do, not with
actual sex determination, but with suitable distribution of the sex
characters to future generations. If both sex characters are present in
parthenogenetic eggs, as appears to be the case in aphids and
phylloxera, dominance of one or the other must be determined by
conditions external to the chromosomes, for we have both sexes at
different points in the same line of descent without either reduction or
fertilization.

Wilson suggests as alternatives to the chromosome sex determinant theory
according to Mendel's Law, (1) that the heterochromosomes may merely
transmit sex characters, sex being determined by protoplasmic conditions
external to the chromosomes; (2) That the heterochromosomes may be
sex-determining factors only by virtue of difference in activity or
amount of chromatin, the female sex chro