C. B. DAVENPORT 



is not to state that all of the elements out of which such charac- 

 ters arise are located there but that something that plays a pre- 

 dominating rdle in the development of such characters is located 

 there. The male fowl is positively homozygous, or, as I prefer 

 to call it, duplex, in respect to the sex determiner just because it 

 has, probably, two sex-chromosomes or their equivalent in the 

 somatic cell. Consequently each of its sperm-cells contains a 

 sex-chromosome and, hence, a determiner for each independent 

 sex-limited character. The female fowl, on the other hand, is 

 heterozygous, or simplex, in respect to the sex-determiner just 



Gametes of the 

 male ; all carry a sex- 

 chromosome with its 

 determiner. 



Gametes of the 

 female; only half 

 carry a sex-chromo- 

 some. 



because it has, probably, one sex-chromosome or its equivalent 

 to the cell. Consequently, only half of the matured eggs of a 

 hen have a sex-chromosome and, hence, a determiner for each 

 independent sex-limited character. The other eggs lack the 

 determiner for any sex-limited character. ^ 



The consequences of the theory outlined above may best be 

 shown graphically and this I have attempted in fig. A. The 

 circles represent sex-chromosomes and the included figures are 



1 It must be admitted that, while the parallelism of cytological and experimental 

 results has been shown in several cases, yet in the only published observations on 

 the spermatogenesis of the fowl (Guyer, '09, p. 579) it is concluded that in the cell 

 division that gives rise to the definitive sperm cells "the odd chromosome passes 

 undivided to one pole in the vast majority of cases." It must, however, be con- 

 sidered that the chromosomes of the testis of the fowl are particularly crowded 

 and hard to disentangle and one is justified in awaiting a confirmation of Guyer's 

 results before attempting to bring them into harmony with the theory here 

 adopted or rejecting a theory which explains practicalh' all the results of experi- 

 mental breeding of sex-limited characters. 



