Lnternal Factors of Sex Determination 423 
eggs and sperm contain both potentialities, and which is realized 
is determined by some internal relation that regulates the domi- 
nance of one or the other sex. 
This idea leads to the following suggestion: — In Mendelian 
hybrids of the first generation, in which two contrasted charac- 
ters are present, it is assumed that when the germ-cells are pro- 
duced half of them show already the dominance of one character 
and the other half of the other character. This leads, after hap- 
hazard matings, necessarily to the three types that appear in the 
second generation. In the case of sex only two types are pro- 
duced, hence it is not possible to imagine that two kinds of eggs 
and two kinds of spermatozoa are produced.! It seems, there- 
fore, logical to conclude that the condition that leads to the de- 
velopment of the alternative characters may exist in the egg 
alone (as for the male bee), or in the sperm alone (as for certain 
hemiptera), or by the combination of egg and sperm, as for the 
female bee. In the last case the conditions that will lead to the 
development of the male are found in the unfertilized eggs; but 
the addition of the sperm brings in a new condition that leads 
to the development of the female — not that the spermatozoa of 
the bee are female or even female-producing, but that the com- 
bination of egg and sperm is female-producing. It is conceiv- 
able that a non-nucleated egg of the bee fertilized by a sperm 
would produce a male, the conditions being the same as in the 
unfertilized egg. 
Admitting that all eggs and all sperm carry the material basis 
that can produce both the male and female, the two conditions 
being mutually exclusive when development occurs, the imme- 
diate problem of sex determination resolves itself into a study 
of the conditions that in each species regulate the development 
of one or the other sex. It seems not improbable that this regu- 
lation is different in different species, and that, therefore, it is 
futile to search for any principle of sex determination that is 
universal for all species with separate sexes; for while the 
1The assumption of reciprocal fertilization being rejected provisionally for 
reasons already given. 
