176 THE SECOND-CHROMOSOME GROUP 



was made, such that a certain class of daughters should all have the 



composition— — r r-- Seven of the eight daughters tested 



had this expected composition, but one (No. 3464) gave only offspring 



corresponding to the composition — - — j -r—. That is, the gene 



for lethal 9 was found to be not in the chromosome in which it entered 

 the zygote, but in the homologous chromosome derived from the other 

 parent. As in the truncate X black case, this transmigration took 

 place after fertilization and so early in the embryonic history that all 

 the germ-cells were descended from this altered cell. A significant 

 feature of this case is that while the change must be described super- 

 ficially as double crossing-over, this double crossing-over occurred 

 within a region only 10 units long — a space shorter than that in which 

 double crossing-over of the ordinary type has ever been detected, even 

 in certain regions of the autosomes in which double crossing-over is 

 relatively frequent. 



OTHER MUTATIONS. 



Tw'o new mutations were found and two old ones recurred in these 

 back-cross experiments on the linkage of purple and vestigial. 



"Kidney" eye-shape, a third -chromosome recessive, was found in 

 B. C. culture B 102, June 26, 1912 (table 27). This mutant, the 

 first affecting the shape or texture of the eye, was considerably used 

 in the early days (see Morgan, 1914, and Bridges, 1915), but has now 

 been superseded by mutants less variable and easier to classify. 



In culture B 39.2 it was noted, July 26, 1912, that several wild-type 

 flies had more bristles on the thorax than the regular number, ^. e., 4. 

 Later it was found that such extra-bristled flies were occurring in 

 small proportions in all four sister cultures, from which it would appear 

 that the mutation was a recessive, introduced through the purple- 

 vestigial stock used twice in the experiment. The extra bristles 

 occurred among all classes in the experiment indifferently, which 

 would seem to indicate that the gene was not second-chromosome, 

 since if it were the extras should have been relatively more frequent 

 among the purple vestigials. The number of extra bristles varied 

 from 1 to 4, the highest total bristle number observed being 8. Extra 

 bristles were also observed to be frequent in two or three other stocks. 

 A stock throwing extra thoracic bristles derived from B 39.2 was main- 

 tained by rough mass selection for some time and was finally given to 

 Mr. E. C. MacDowell to be used as the basis of rigorous selection experi- 

 ments (MacDowell, 1915). As the result of a survey of all stocks 

 known or suspected to contain extra bristles, MacDowell chose a cer- 

 tain wild stock as the most favorable starting-point for his selection. 



