x] Cytological Evidence as to Sex 189 



this is the fact that while the number of chromosomes in 

 the male cells of the species concerned is either n, or n i, 

 the number in the female cells is n. The male cells there- 

 fore which have n chromosomes on uniting with an ovum 

 in fertilisation make up the zygotic number to in ; but the 

 n i spermatozoa can only make up the zygotic number to 

 27z i, forming thus a male. The germ-cells of the male 

 again divide in gametogenesis to form cells with n, and 

 cells with n i chromosomes as before. 



In such cases it can scarcely be stated yet that the 

 accessory chromosome is the cause of femaleness, for con- 

 ceivably it may be a feature associated with that cause ; but 

 the evidence must be taken with confidence as the long- 

 expected proof that sex is determined, in this case at least, 

 by gametic differentiation. The female in these Insects 

 must then be regarded as homozygous in sex, and may 

 be represented as DD, while the male is heterozygous and 

 may be represented as DR. Such a result accords well 

 with the general conclusions to which breeding experiments, 

 on the whole, point. For though great disparities between 

 the numerical proportions of the sexes occur in certain 

 matings, these disparities seem to be obliterated in suc- 

 ceeding generations. If the one sex were homozygous and 

 the other heterozygous such impermanence of the numerical 

 divergences is what we might naturally expect. Neverthe- 

 less the extraordinary fact remains that while the cytological 

 evidence suggests that the male is heterozygous, equally 

 cogent evidence from breeding in other types indicates 

 \hefemale as the heterozygous sex. 



T. H. Morgan (202)* has lately carried the discussion a 

 stage further. It is well known that in the case of animals 

 having a series of parthenogenetic female generations, such 

 as Aphis, Daphnia, &c., when fertilisation does take place, 

 the result of such fertilisation is always a female. In a 

 Phylloxera Morgan has found that the spermatids are of 

 two kinds, those which contain an accessory chromosome, 

 and those which do not. The spermatids lacking the acces- 

 sory body degenerate, and consequently only those provided 

 with it take part in fertilisation. The fact that the results 



* Since confirmed by von Baehr (7) for Aphidae. 



