254 Sturtevant. 



more independent genes than haploid number of cliromosomes (7, ac- 

 cording to Cannon '03b and others). Lock ('06) on the other hand, has 

 taken the argument as valid: but has fallen back on the CoRRENs- 

 DE Veies conception of an interchange between homologous chromosomes 

 as an explanation of the difficulty. 



In connection with the idea of interchange between homologous 

 chromosomes, Coreens ('Oä) and Lock ('06) made an important suggest- 

 ion, which, however, was not taken up until arrived at independently 

 by MOEGAN ('lib, 'lie): the phenomena of linkage between genes must 

 be due to a failure of this interchange. We shall come back to this 

 side of the question after considering briefly the matter of sex deter- 

 mination. 



Sex Chromosomes and Sex Linkage. 



The first attempt at connecting any given character with a definite 

 chromosome was made by Mc Clung ('02). This author suggested that 

 the "accessory chromosome" observed in several insects may be a sex 

 determiner. Stevens ('05) and Wilson ('05) soon showed this to be 

 actually the case, although not quite in the manner which Mc Clung 

 had suggested. As a result of the immense amount of work since done 

 on tliis subject there can no longer be any doubt that in many arthro- 

 pods, nematodes, and probably mammals and echinoderms there is in 

 the female a pair of equal chromosomes, while in the male this pair is 

 represented either by a single chromosome or by an unequal pair; that 

 there are two classes of sperm but only one of eggs: and that the two 

 classes of sperm, differing as regards this chromosome pair, are, respect- 

 ively, male-producing and female-producing. 



MOEGAN ('10b) next showed that in Drosophila ampelophila there 

 is a factor for color in the eyes (the W of this paper), the distribution 

 of which is exactly that of the X chromosomes previously found in tMs 

 species by Stevens ('08). Each female is duplex for it, and each Qgg 

 is simplex. Each male is simplex, and transmits it only through his 

 female-producing sperm. There are now known to be similar cases in 

 man, (Morgan '11a, Davenpoet '11, Newman '13 &q..) and in the cat 

 (DoNCASTEE '11, '13, Little '12b). The cytological evidence for man 

 is conflicting, but Guyeb ('10) and Winiwarter ('12) are agreed that 

 sex-chromosomes having the required distribution are present. 



The large number of sex-linked genes now known in Drosophila 

 furnish a means for testing the hypotheses outlined above, since if there 



