STUDIES IN THE EXPEEIMENTAL ANALYSIS OF SEX. 589 
and before any visible differentiation of the eggs into male- 
producing and female-producing forms has occurred, so that 
this differentiation may be the result of fertilisation and not 
due to the inherent heterozygotism of the female. In the 
case of Hydatina,* as clearly shown in R. C. Punnett’s 
interesting' paper (10), and in the Cladocera, there is no doubt 
that the female can give rise parthenogenetically to both males 
and females, so that in these cases the female sex is certainly 
heterozygous; but we have no certain means of judging 
whether it is the female eggs alone which are capable of 
being fertilised. 
Interesting and suggestive as the evidence drawn from 
breeding experiments and from the cytology of maturation is, 
it appears that the most cogent and unassailable evidence for 
sex heterozygotism is afforded firstly by Inachus parasitised 
by Sacculina, in which the male sex is proved to possess the 
secondary and primary female characters in a latent state, 
and secondly by the Cladocera, in which the female sex is 
proved to be heterozygous owing to the parthenogenetic 
females giving rise to both males and females. 
The foregoing arguments and considerations have shown 
us that while a number of facts definitely support the half- 
hybrid Mendelian theory of sex, there is nothing which 
definitely controverts it. The theory has the merit of being 
a simple one, and it accounts for the facts without the 
necessity of making any additional assumptions such as that 
of selective fertilisation, an assumption which may in the 
future prove necessary, but which would seriously impair 
the validity of Mendelian analysis. 
And it may be retnarked that the half-hybrid theory of 
sex not only alters our view of the sexual constitution of 
animals and plants, but it indicates, if it is well founded, the 
real ground upon which the problem of sex must be attacked. 
This, as has been already stated, is the inquiry as to the 
physiological conditions under which one sex or the other 
gains the upper hand, i.e. becomes dominant in a heterozygous 
individual which contains potentially the elements of both sexes. 
