98 SCIENCE PROGRESS 



writer 1 to formulate an almost exactly similar hypothesis at 

 almost the same time. But it will be seen that later experi- 

 ments with moths suggest that a slightly different explanation 

 of the facts is more probable. 



The essence of Wilson's hypothesis is that the sex-deter- 

 minants behave as Mendelian characters, segregating from one 

 another in gametogenesis (at the reduction division), that female- 

 ness is dominant, and that there is selective fertilisation ; so 

 that a male-bearing egg is fertilised by a female-bearing sperma- 

 tozoon. Suggestions closely similar to these were put forward 

 by Castle 2 on quite different grounds in 1903. Castle collected 

 a quantity of evidence from breeding experiments and from 

 what is known with regard to parthenogenetic reproduction. 

 He supposed that every individual arising from a fertilised 

 egg is heterozygous (hybrid) in respect of sex and that 

 segregation takes place at the second maturation division, so 

 that half the gametes bear maleness, half femaleness. Male- 

 bearing eggs conjugate with female-bearing spermatozoa and 

 vice versa ; but dominance is alternative, so that roughly half 

 develop into each sex. In most parthenogenetic animals only 

 one polar body is produced in eggs which will not be fertilised ; 

 in these cases femaleness is always supposed to dominate. 

 Since with only one polar division no segregation takes place, 

 the offspring are females. If in a parthenogenetic species two 

 polar bodies are produced the offspring are commonly males, 

 since the female determinant is supposed to be eliminated with 

 the second polar nucleus. A further valuable suggestion was 

 that the male and female determinants might be " coupled " 

 with certain body characters, either invariably — so explaining 

 sexual dimorphism — or frequently, by which is explained the 

 general association of one variety with one sex, another with 

 the other, in the offspring of certain crosses. Wilson has 

 since 3 observed coupling of ordinary with idiochromosomes, 

 which may be connected with this phenomenon. 



Castle's suggestive paper has stimulated much work on the 

 maturation of parthenogenetic species but his hypotheses do 

 not always hold good. For example, it is now known that 

 parthenogenetically produced males in the Aphides arise from 



1 Doncaster and Raynor, Proc. Zoo. Soc. 1906, i. p. 125. 



2 "The Heredity of Sex," Bull. Mus. Comp. Zoo. Harvard, xl. p. 189. 



3 Science, May 17, 1907, p. 779. 



