1911] SHULL—REVERSIBLE SEX-MUTANTS 359 
whether the female of Lychnis dioica is a positive, neutral, or 
negative homozygote, or whether the synaptic mate of the female 
gene is qualitatively male or not. ‘The matter has been considered 
at such length only because it is important that no unwarranted 
conclusions should be drawn from the configuration of the chromo- 
somes in any given case. 
There appears to be no very strong evidence at present that the 
chromosomes are the representatives or producers of particular 
Mendelian unit characters, though attempts have been made a 
number of times during the past decade to identify them as such. 
On the other hand, there is still no positive and complete demonstra- 
tion that the chromosomes are not the determiners of the Mendelian 
characters, and until this demonstration is provided, the relation 
of the chromosomes to the unit characters must be kept open. 
Whether the chromosomes are responsible directly for sex may well 
remain likewise an open question for the present, especially in view 
of the fact that in many animals, and in the few plants which have 
been thus far investigated, no chromosome differences have been 
found to differentiate the sexes. 
There can be no doubt of course that the sex characters are 
associated with chromosome differences in the considerable number 
of animals which have been found to present such differences, but, 
as we have just seen, the nature of this association is not clear. 
Where two types of sperms are found in the male, the one type 
corresponding in its chromosome complex with the single type 
Presented by the eggs, the inference is fully justified that such 
males are heterozygous and the females homozygous in respect 
to sex, whether one or more chromosomes be the sex-determiner, 
or whether these chromosomes are merely symptomatic of other 
fundamental differences which are the true sex-determiners; and 
vice versa, when two types of eggs having different chromosome 
§toups are found in the female, one of which agrees with the only 
type found in the sperms, the inference is fair that the female is 
heterozygous and the male homozygous in respect to sex. So 
consistent have been the results in those species in which both male 
and female germ cells have been investigated, that it has not seemed 
improper to assume that in any given species the one sex will have 
