The President's Address. By J. Arthur Thomson. 15 



o 



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 ; and that the association of 

 two such elements leads to a reaction which sets free the female 

 character. 



27. Here we may refer to Professor E. B. Wilson's proposal to 

 draw a distinction between sexual predetermination and sexual 

 predestination. " The definitive determination of maleness or 

 femaleness only occurs when all the factors necessary to their 

 production have been brought together. This may be effected 

 before fertilization (' progamic determination ' of Haecker), but may 

 also first ensue upon union of the gametes (' syngamic determina- 

 tion '). Thus one rmiy suppose that all the sexual eggs of a queen- 

 bee and of Maupas' Hydatina are predestined towards maleness, but 

 this is reversed by fertilization when determination occurs." 



D. — Are maleness and femaleness Mendelian characters ? 



28. We have seen that many facts point to the conclusion that 

 the sex-cells have definite sexual tendencies or predispositions. 

 The question has naturally arisen whether these tendencies to 

 male-production or female-production correspond to two contrasted 

 Mendelian characters, to the dominant and recessive members of a 

 pair of allelomorphs. This Mendelian interpretation of sex, first 

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

 Bateson, and others. As Professor Wilson points out, the inter- 

 pretation 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 hetero- 

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

 that the female is the heterozygote, the male being a homozygote 

 recessive (Bateson)." 



As Professor 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. 



Professor 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 character, 

 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 fertilized 

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

 by the other half produce female-plants. 



