294 



THE SECOND-CHROMOSOME GROUP 



these two classes of offspring received second chromosomes that came 

 from the two original stocks. If those original stocks differed in 

 second-chromosome modifiers the two classes should differ in mean 

 bristle-number. This has, in fact, been found to be the case (see below) . 

 But if an FI female is used, these differences should be less than in the 

 above case, provided the modifier, or modifiers, crossed over from 

 speck. This result has also been obtained, as will appear from table 

 138, which shows the excess in mean bristle-number of the not-specks 

 over the specks among the dichaete offspring from back-crosses of the 

 type described above. The two values in any one line are from the 

 same combination of stocks, and are therefore available for comparison 

 of male tests with female tests. 



TABLE 138. 



These data demonstrate the existence of one or more second-chromo- 

 some modifiers for dichsete bristle-number, and show that at least one 

 such modifier crosses over from speck. 



Further details and conclusions of this selection experiment and a 

 general discussion of the subject are given by Sturtevant in Carnegie 

 Institution of Washington Publication No. 264. 



DACHSOID. 



(Text-figure 86.) 



ORIGIN OF DACHSOID. 



In testing for the presence of a third-chromosome cross-over variation 

 reported to be present in the eosin stock, Sturtevant out-crossed two 

 eosin males separately to females from the stock containing the third- 

 chromosome recessives sepia, spineless, kidney, sooty, and rough. 

 Several of the daughters from each of these matings were back-crossed 

 to males from the multiple recessive stock. This experiment is dis- 

 cussed by Sturtevant in the paper appearing herewith (Part III) . From 

 both series brother-sister pairings of the back-cross type were continued 

 through several generations. "Dachsoid," a new mutant wing-type, 

 appeared in four out of seven of the inbred cultures of F 2 in one strain 

 (from 2568a). The first of these was observed, February 9, 1917, in 



