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. Tne 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 

 groups 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 



