254 CHARLES W. METZ 



ported by much evidence other than that from the Diptera. 

 Two cases bear such a resemblance to those described in the 

 Diptera that they will be briefly noted here. The most striking 

 is that described by Wilson ('10) in Metapodius, certain individ- 

 uals of which possess a supernumerary Y-chromosome or a 

 supernumerary 'm-chromosome'. The extra chromosome in 

 these specimens always "behaves according to its own kind" 

 (p. 69), exhibiting a definite relation to those of its own kind, 

 but to no others, in the pre-reduction stage of maturation. ^^ 

 Similarly Miss Woolsey ('15) has found that in a certain speci- 

 men of Jamaicana subguttata two small chromosomes act as the 

 synaptic mate of one large bipartite chromosome which has appar- 

 ently arisen by the union of two chromosomes corresponding 

 respectively to the two with which it associates. These cases, 

 like those of pairing in the Diptera, are readily explained upon 

 the assumption that the association depends upon a qualita- 

 tive likeness between corresponding chromosomes, but are diffi- 

 cult to interpret otherwise. 



At first sight the conclusion that only qualitatively similar 

 chromosomes associate in pairs might seem to be contradicted 

 by the pairing of the unequal XY chromosomes in the males; 

 but the contradiction, I believe, is apparent only. The studies 

 .^ of Stevens on Coleoptera and Diptera, and of Wilson, Payne and 

 others on Hemiptera indicate that the XY pair when present 

 has arisen either from an XY, XY pair, one member of which 

 has lost its X-chromatin, or from a Y, Y pair, to one member of 

 which X-chromatin has become attached.^'' In either case 

 the X-chromosome of the Diptera may be looked upon as a Y- 

 chromosome with X-chromatin added to it; and upon the view 

 that similarly constituted chromosomes associate together the 

 Y-portion of X would be expected to associate with the true 



^^ See footnote 20, page 264. 



^' ". . . . we may, accordingly, think of the XY-pair as being essentially 

 a YY-pair with one member of which the X-chromatin is associated." (Wilson 

 '11, p. 87.) Genetic work on Drosophila indicates that the Y-chromosome in 

 this fly is inactive (i. e. no factors have been found in it), but this does not neces- 

 sarily mean that it is physico-chemically different from the Y-part of X. 



