THE FACTORS WHICH DETERMINE SEX 637 



inapplicable to Ampliibians. The alternative theory that sex 

 depends on the position of the testis from which the fertilising 

 spermatozoon was derived has been negatived by Copeman ^ as 

 a result of an experimental investigation upon rats. 



Castle's Theory. — Bateson ^ was the first to suggest that the 

 MendeUan laws are applicable to sex-heredity. This suggestion 

 has been elaborated by Castle ^ into a theory which is based on 

 the idea that sex is determined during the process of maturation, 

 when the male and female gametes are believed to undergo 

 differentiation. Thus, in the case of the ovum, the male or 

 female element is supposed to be ejected in one of the polar 

 bodies, while a similar process is thought to occur in spermato- 

 genesis, excepting that in the latter case all the products of 

 division become functional gametes. According to this hypo- 

 thesis the ordinary sexual individual is regarded as comparable 

 to a Mendelian hybrid. It is clear, however, that the ordinary 

 MendeUan interpretation requires modification if it is to be 

 applied to the phenomena of sex, since hermaphrodite in- 

 dividuals do not occur in accordance with the usual formula 

 which assumes a gametic segregation and three kinds of 

 conjugation according to the law of probabihties : — 



Spermatozoa . 50 per cent, male + 50 per cent, female. 

 Ova . . 50 per cent, male + 50 per cent, female. 



Eesidt after ) g^, pgr cent. mm. + 50 per cent, mf . + 25 per cent. S. 

 conjugation ) 



If this result actually happened, hermaphrodite individuals 

 (mf.) would be twice as common as individuals belonging to 

 either sex. Castle assumes, therefore, that male spermatozoa 

 are capable of conjugating with female ova only, and that 

 female spermatozoa can conjugate with male ova only. The 

 actual determination of sex in the zygote is supposed to 

 depend upon whether the male or female character is dominant. 



Dominance, according to this theory, in dioecious forms, is 



' Oopeman, " Sex Determination," Phys. Soc, May, 1908. 



2 Bateson, Report to the Evolution Oommittee of the Royal Society, I., 

 1902. 



' Castle, "The Heredity of Sex," Bulletin of the Museum of Comparative 

 Zoology, Harvard, vol. xl., 1903. 



