CHROMOSOMES OF INDIAN RUNNER DUCK. 347 



remain in the female determining gametes, would be concerned 

 only with sex-determination. 



If the large unpaired chromosome in the cells of the female is 

 one of a complex of three that go to make up the sex-determining 

 mechanism, the condition in the duck might then be a case similar 

 to that in Phragmatobia, one of the Lepidoptera, which was 

 described by Seiler (1914). 



In Phragmatobia during polar body formation a single element 

 was seen to separate from a large heteromorphic element that 

 consisted of two unequal chromosomes yoked together. After 

 separation the heteromorphic element fragmented into two un- 

 equal chromosomes at one pole. As the result, half of the eggs 

 got the large single element and half got the two unequal chromo- 

 somes. 



If the duck be like Phragmatobia, the unpaired chromosome 

 here designated as Z might be thought comparable to the large 

 element in Phragmatobia and some two of the other large chromo- 

 somes, here designated W, w, might be comparable to the hetero- 

 morphic element in Phragmatobia which was seen to be distributed 

 to one pole in maturation and to break into two chromosomes. 



SEX-DETERMINATION AND SEX-LINKAGE. 



Wilson (1925) is of the opinion that "all the difficulties" in 

 regard to an explanation of the function of sex-chromosomes in 

 sex-determination " disappear is we assume that in any particular 

 species there is but one kind of X-chromosome, in itself neither 

 male-determining nor female-determining, but so adjusted to the 

 general mechanism of development that when single it swings 

 development toward the male side, when double toward the 

 female side. This view essentially quantitative ascribes to the 

 egg the capacity to produce either the female or the male, ac- 

 cording to the presence of more or less of the X substance." 



On the other hand, Bridges (1922) from his study of triploids 

 in Drosophila concludes that "both sexes are due to the simul- 

 taneous action of two opposed sets of genes, one set tending to 

 produce the characters we call female and the other to produce 

 the characters called male." According to his hypothesis there is 

 a preponderance of male tendency genes located in the autosomes 



