154 SEX-DETERMINATION [CH. 



The first type of sex-determination by means of a differ- 

 ence of the chromosomes in the egg at the beginning of 

 development is thus that in which a male-determining egg 

 has n chromosomes and the female-determining egg of the 

 same species 2n. This type is best known in the Hymenop- 

 tera. It probably occurs also in the Rotifers, in which it 

 seems fairly clear that males are produced from partheno- 

 genetic eggs having two maturation divisions, giving the 

 haploid number; while females arise either from partheno- 

 genetic eggs which have only one maturation division, 

 leaving the diploid number of chromosomes in the egg, or 

 from eggs which undergo a double (reducing) maturation 

 division and are then fertilised. In both Hymenoptera and 

 Rotifers, therefore, fertilised eggs appear always to yield 

 females. 



It will be convenient to postpone consideration of the 

 other chief group of parthenogenetic species that including 

 the Aphids and parthenogenetic Crustacea in which both 

 sexes may arise from unfertilised eggs having only one 

 maturation division, and in which, as in the Hymenoptera 

 and Rotifers, fertilised eggs always produce females. The 

 phenomena presented by this group will be more easily 

 understood after some account has been given of the rela- 

 tions between chromosomes and sex in animals in which the 

 eggs all require fertilisation. 



In the account of the maturation of the germ-cells, and 

 of fertilisation, given in Chapters V-VIII, it was assumed 

 that the chromosome-groups of the male and female, and 

 hence of the sperm and egg, were alike. In a large number 

 of animals this is to all appearance true, but in recent years 

 it has been shown that in many species, belonging to very 

 different groups, such a statement is not strictly correct. 

 Nearly all the chromosomes do accurately correspond in the 



