194 JOHN S. DEXTER. 



double chromosome on which any factor should be thrown by the 

 twist. As Morgan points out, the chances of separation are not 

 so great for factors which lie near each other as for those far from 

 each other, though coupling would not necessarily imply that 

 the factors should lie in one segment of the twisted chromosome. 

 According to this idea the factors for wing length, as explained 

 by Morgan in the paper in the Journal of Experimental Zoology 

 for November, 1911, may not be closely approximated to those 

 for eye color and body color. 



SUMMARY. 



1. The factors for red and for white eye color and for normal 

 and yellow body color in Drosophila have been the subjects of 

 investigation in these experiments. 



2. In the F 2 generation the factors for eye color and body 

 color appear to a great extent associated in the same combinations 

 .that were present in the grandparents, the interchanging taking 

 place only once to nearly eighty cases where there is no inter- 

 changing. 



3. The absence in a fly of certain characters found in the normal 

 wild fly, seem to render it less fertile, or at least such flies hatch 

 in smaller numbers than do those in which such characters are 

 present. 



4. These facts are in accord with the principles and theories 

 advanced by Morgan, and can be explained on a theory based on 

 (i) the relative positions of factors in the chromosomes, (2) the 

 twisting of homologous chromosomes about each other in gameto- 

 genesis, and (3) their subsequent splitting in one plane. 



5. Further cytological evidence bearing on the above theory 

 is much to be desired. 



