58 MENDELISM AND 



sorts, one with N chromosomes, the other with N I 

 or N 2 chromosomes. On fertilisation two kinds 

 of zygotes are formed, female-producing eggs with 

 2N chromosomes, and male-producing eggs with 

 2 N I or 2N 2 chromosomes. There is also evidence 

 that in some cases, e.g. the sea-urchin, the female 

 is heterozygous, forming gametes, some with N 

 and some with N+ chromosomes, while the male 

 gametes are all N. Fertilisation then produces 

 male-producing eggs with 2N chromosomes, female- 

 producing with 2N+. 



Such is the summary given by Castle in 1912. 1 

 It will be seen that he treats the differences as purely 

 quantitative, mere differences in the number of the 

 chromosomes. Professor E. B. Wilson, however, 

 who had contributed largely by his own researches 

 to our knowledge of sex from the cytological point 

 of view, had already published, in 1910, 2 a very 

 instructive resume of the facts observed up to that 

 time. The important fact which is generally true 

 for insects, according to Wilson, is that there is a 

 special chromosome or chromosomes which can be 

 distinguished from the others, and which is or are 

 related to sex differentiation. This chromosome, 

 to speak of it for convenience in the singular, has 

 been variously named by different investigators. 

 Wilson called it the ' X chromosome,' M'Cluny the 

 4 accessory chromosome,' Montgomery the ' hetero- 

 chromosome,' while the names c heterotropic chromo- 

 some ' and idiochromosome have also been used. 

 For the purpose of. the present discussion we may con- 



1 Heredity and Eugenics, by Castle and Others. University of Chicago 

 Press, 1912. 



8 ' The Determination of Sex.* Science Progress, April 1910, 



