Chromosome Relationships. 9 



characters, or portions of such groups, in the different species. 

 For instance, each of the large non-sex-Hnked groups of characters 

 in D. melanog aster should resemble two of the groups in virilis com- 

 bined, while the sex-linked groups should correspond in the two 

 species. This is only one of the possible hypotheses to cover the 

 case, but it will serve to illustrate the mode of attack. 





» 2 3 



Fig. 3. — Camera lucida drawings of chromosomes of Drosophila virilis. 1 and 2 from ovarian 



cells, 3 from a spermatogonia! cell. 



Such considerations are, of course, based on the assumption that 

 homologous mutant characters may be found in the species concerned. 

 The validity of this assumption is becoming more and more probable 

 as the work progresses (see below), and the question now hinges 

 mainly on whether or not a sufficient number of homologous charac- 

 ters may be obtained for the purpose. 



CHROMOSOMES OF DROSOPHILA VIRILIS. 



The chromosome group of D. virilis, as noted above, consists of 

 6 pairs, 5 of which are rod-like and of approximately the same size, 

 and 1 of which is small and spherical. The small pair normally lies 

 in the center of the figure during metaphase, as shown in the accom- 

 panying figures (fig. 3). In size, position, and behavior this pair 

 agrees with the similar pair in most other species of Drosophila 

 (Metz, 19166). 



The five pairs of rod-like chromosomes have a terminal spindle- 

 fiber attachment and hence are usually arranged radially in metaphase 

 (figs. 1 and 2, Metz, 1916a). As in other species of Diptera, homol- 

 ogous chromosomes are normally associated in pairs in the somatic 

 cells, as well as in the germ cells.' 



In some figures one of the five rod-like pairs appears to be slightly 

 longer than the others, suggesting that it may represent the sex 

 chromosomes. But since no pair is conspicuously dimorphic in the 

 male, this view has not been corroborated. It seems practically 

 certain, however, from both cytological and genetic evidence, that 

 one of the rod-like pairs, and not the small spherical pair, is the 

 sex-chromosome pair. 



1 For a discussion of this feature see Metz, 1916a. 



