THE NATURE AND EXPRESSION OF MEN DELI AN FACTORS 135 



case, for pure mutant stocks constantly show a high percentage of flies 

 with normal abdomens. This variability in abdomen characters has 



FIG. 69. Club-winged Drosophila. At a characteristic unfolded wing pads. At c 

 the absence of spines on the side of the thorax is shown in comparison with the normal con- 

 ditions, b. (From Morgan.) 



FIG. 60. Mutant type of Drosophila ampelophila called abnormal abdomen (the 

 wings have been cut off); a, female; b, male; c, female that approaches the normal type. 

 Development of this character is dependent upon moisture. (From Morgan.) 



been found to depend upon the condition of the food. When the food 

 is moist a high percentage of flies have abnormal abdomens, but when the 

 larvae are raised on dry food nearly all of them have normal abdomens. 



