No. 583] BOLE OF ENVIRONMENT IN DBOSOPHILA 401 



cross-overs are expected in this mixed class by compari- 

 son with the other single cross-over class (YWAb). If 

 these are subtracted, the remainder shows how many of 

 this GrKN class are due to a change from the abnormal to 

 normal. Another point to note is that one of the double 

 cross-over classes, viz., YRX, is likewise subject to addi- 

 tion from the single cross-over class, YKAb, and can not 

 itself be taken as a measure of double crossing over, while, 

 on the contrary, all cases in the other double cross-over 

 class, viz., GWAb, count for their full value. Only two 

 such double cross-overs occurred. 



On the basis of the amount of single crossing over it is 

 possible to calculate, as Sturtevant has shown, the ex- 

 pected number of double cross-overs. The number of 

 the double cross-overs (two) in Table VIII is larger than 

 expected. I repeated (December, 1913) the last experi- 

 ment to test the question because abnormal arrangement 

 of the rings of the abdomen is not a very rare occurrence 

 and may sometimes be the result of injury to the larva or 

 to the pupae, or in still other cases may be due to other 

 mutations, some of which will be described later. The 

 abnormal mutation itself occurs not infrequently under 

 conditions precluding contamination. In repeating the 

 experiment extreme care was taken not to classify any fly 



TABLE VIII 

 Parents: GRAb £ by YWN$ 

 P t : GRAb $ — YWN $ 



