THE NATURE AND EXPRESSION OF MENDELIAN FACTORS 135 
case, for pure mutant stocks constantly show a high percentage of flies 
with normal abdomens. This variability in abdomen characters has 
Fic. 59.—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.) 
Fie. 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 
larve are raised on dry food nearly all of them have normal abdomens. 
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