236 HEREDITARY TRANSMISSION [CH. 



unpaired or accompanied by a dissimilar mate (T), while the 

 female has two similar Jf-chromosomes. On the other hand, 

 in Lepidoptera (certainly in two cases) and in Birds (by 

 inference from the nature of their sex-limited transmission) 

 the male has two similar sex-chromosomes and the female 

 one or a dissimilar pair 1 . Two chief hypotheses have been 

 suggested to explain their manner of action. One was that 

 in the former group the two Jf-chromosomes of the female 

 bore respectively the factors for maleness and femaleness, 

 femaleness being dominant, and the single ^-chromosome 

 of the male bore only maleness. There would thus be two 

 kinds of eggs, which may be called M and F, and two kinds 

 of spermatozoa, M and (0 representing the absence of an 

 Jf-chromosome). If then there is selective fertilisation of 

 such a kind that F-bearing eggs are only fertilised by M- 

 bearing spermatozoa, and M -bearing eggs only by 0- 

 bearing spermatozoa, two kinds of zygotes,FM and MO, will 

 be produced, and these will become females and males of the 

 same constitution as their parents. In the Lepidoptera and 

 Birds maleness instead of femaleness must be supposed to 

 be dominant, and the male would have the constitution 

 MF, the female FO ; /'-bearing eggs would then be fertilised 

 by M-bearing spermatozoa, and 0-bearing eggs by /"-bear- 

 ing spermatozoa. The great objection to this hypothesis is 

 that it involves the assumption of selective fertilisation, for 

 which there is practically no evidence, and it is now gene- 

 rally abandoned in favour of a somewhat simpler concep- 

 tion that there are not two factors, for maleness and 

 femaleness, but that the sex depends on the presence of a 



1 GUYER maintains that in the Fowl there is an unpaired chromosome in 

 the male. In view of the great difficulty in observing the chromosomes 

 accurately in the spermatogenesis of birds, I venture to doubt the correctness 

 of his statement, which has not been confirmed. 



