r;7G 



THE AMERICAN NATURALIST [Vol. XL VII I 



3 : 1 ratio. Selecting again from the winged flies, the proc- 

 ess could be repeated indefinitely. The only difficulty lay 

 in the fact that no visible character differentiated the 

 heterozygous from homozygous winged flies, and conse- 

 quently all matings had to be made in pairs taken at 

 random, with the result that about 56 per cent, of the 

 cultures were rendered worthless. In actual practise large 

 numbers were mated in pairs, and then all discarded save 

 those producing apterous. 3 This was the method used in 

 keeping up stock. 



To obtain the necessary combination of apterous with 

 other mutant factors, winged offspring from apterous- 

 throwing parents were mated in pairs to flies of the 

 desired stock. One third of the normals from apterous 

 stock were pure for the normal allelomorph of apterous 

 and rendered worthless all matings in which they were in- 

 volved; but the other two thirds were heterozygous for 

 apterous, and when crossed with the desired stock gave 

 in F 2 some apterous offspring. If the F x flies were bred 

 en masse, approximately 15 winged to 1 apterous were ob- 

 tained, but if bred in pairs, certain pairs (those in which 

 both members were heterozygous for apterous) gave 3 

 winged to 1 apterous. The latter method was the one 

 actually used in most cases. In this manner the same end 

 result was attained as would have been secured by using 

 apterous individuals in crosses with other stocks, the 

 only difference being in the amount of labor involved in 

 making up a larger number of cultures. Both kinds of 

 crosses were, in fact, used, as will be seen below. 



The use of symbols in this paper follows the system 

 recently adopted by Morgan and other students of Droso- 

 phila (Morgan, 1913, a and b). That is, for any pair of 

 allelomorphic characters a capital letter is used to indicate 

 the dominant, and a small letter the recessive factor— the 

 symbol being taken from the name of the mutant. Since 

 the apterous character is recessive, the symbols for the 



out the heterozygous individuals. 



