MENDELISM AND SEX 199 



polar bodies in these forms do not in fact enter into the 

 composition of any part of the body, yet the sexual relations 

 are the same as in the bee. This difficulty seems to me 

 to constitute a formidable obstacle not only to Castle's 

 hypothesis, but to the whole Mendelian interpretation. 



" The second hypothesis is that of Correns, which assumes 

 the male to be a sex-hybrid while the female is a homozygous 

 recessive. The fertilisation formulas are accordingly 



Egg 9 -}- spermatozoon <J> =zygote $ $ (female) 

 Egg ? -f spermatozoon < = zygote ( $ ) <$ (male). 



This conclusion is based on the following beautiful experi- 

 ments. Crosses between monecious l and diecious flowering 

 plants show that the monecious character behaves like a 

 ' unit character ' which is recessive to the diecious. If 

 reciprocal crosses be made between the monecious Bryonia 

 alba and the diecious B. dioica, the results are as follows. 

 Female dioica crossed with the male alba gives all females. 

 The reverse cross gives half males and half females. From 

 the fact that all the offspring of female dioica X male alba 

 are females, Correns concludes that all the eggs bear this 

 tendency, which dominates the monecious character of the 

 male parent. In the reverse cross the diecious character 

 again dominates, but in this case is derived from the male 

 parent. The appearance of the two sexes in equal numbers 

 must therefore mean that half the pollen grains bear a 

 dominant male tendency and half a recessive female. A 

 similar result is reached by Boll by experiments of quite 

 a different character on the hemp, but the proof seems to 

 me less cogent. 



" Correns's experiments are of admirable ingenuity, and 

 his results seem at first sight to be open to but one 

 conclusion. His interpretation renders the hypothesis of 

 selective fertilisation unnecessary ; for the chance fertilisa- 



1 A plant (or animal) is said to be diecious, when individuals produce only 

 sperms or only ova, that is, when male and female individuals are separate. 

 Monecious individuals produce both sperms and ova, the same individual thus 

 performing the functions of both sexes. 



