496 HEREDITY AND SEX 



that the X-element contains factors (enzymes or hormones ?) 

 that are necessary for the production of both the male and the 

 female characters ; that these are so adjusted that in the presence 

 of a single X-element the male character dominates, or is set 

 free ; while the association of two such elements leads to a 

 reaction which sets free the female character. 



§ 8. Fourth Theory : — That Maleness and Femaleness are 

 Mendelian Characters 



A Mendelian interpretation of sex, first suggested by 

 Strasburger, has been developed by Castle, Correns, Bateson, 

 and others. As Prof. Wilson points out, the interpretation 

 has taken " three forms, which exhaust the a 'priori possibilities. 

 These are, first, that both sexes are sex-hybrids, or heterozygotes 

 (Castle) ; second, that the male alone is a heterozygote, the 

 female being a homozygote recessive (Correns) ; third, that the 

 female is the heterozygote, the male being a homozygote re- 

 cessive (Bateson)." 



As Prof. Wilson has shown, each of these forms of the theory 

 has its special difficulties, which seem to be most serious in the 

 case of the first. 



Prof. Correns's theory was based on beautiful experiments 

 in crossing dioecious and monoecious forms of Bryony, which 

 showed that the monoecious condition behaves as a unit char- 

 acter, which is recessive to the dioecious. 



The experiments made by Correns go to show that the pollen- 

 grains of the dioecious Bryony, though apparently all alike, must 

 be regarded as of two kinds in equal numbers — male-producing 

 and female-producing. What immediately arise, as a matter of 

 fact, are the rudimentary male prothallia, which produce the 

 reproductive gametes or pollen-nuclei, and the egg-cells fertilised 

 by half of these produce male plants, while the egg-cells fertilised 

 by the other half produce female plants. 



