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THE AMERICAN NATURALIST 



[Vol. LVI 



scattered more or less at random through the genus — 

 which does not conform to such a view unless this t\T3e 

 of chromosome group be considered primitive and the 

 forerunner of several other types. 



These considerations indicated the need of a compara- 

 tive study of different species possessing this type of chro- 

 mosome group, in addition to the studies already being 

 made on species having different types of groups. Since 

 the species can not be hybridized (or have thus far refused 

 to hybridize — with one exception considered below), it is 

 necessary to make cytological and genetical studies of 

 them individually. This of course limits the comparison 

 to a very few species. 



The present paper supplements our previous one 

 (Lancetield and Metz, 1921) on the sex chromosome re- 

 lationships of wiUisfoni and melanogcister, in which it 

 was shown by means of non-disjunctional flies that the 

 sex chromosomes are not strictly homologous in the two 

 species. In melanog aster the short, rod-like pair is the 

 sex chromosome pair (Bridges, 1916), whereas in willis- 

 toni we find that the rod-like pair is an autosome pair, 

 and that one of the large V-shaped pairs is the sex chro- 

 mosome pair. This relationship is shown in Figs. A 

 and B. 



The genetical study considered here is for the purpose 

 of comparing the constitution of the sex chromosomes by 



