236 THE SECOND-CHROMOSOME GROUP 



VALUATION OF MORULA. 



The locus of morula is the farthest to the right of the workable 

 mutants, and for this reason is valuable, though speck, which is less 

 than 2 units to the left of morula, will probably be used in the majority 

 of cases. There is another reason for the preference of speck over 

 morula in general — that morula would interfere in classification with 

 star, which is the most useful of the second-chromosome mutants, 

 while speck interferes neither with star nor any other second-chromo- 

 some mutant. The female sterility of morula also limits its usefulness 

 in complex experiments. On the other hand, the character morula is 

 exceptionally easy and certain in its separability from the wild-type, 

 being surpassed in this regard by no other s-econd-chromosome mutant 

 except vestigial. Its viability also is excellent. 



APTEROUS (a,). 



(Plate 7, figure 5.) 



ORIGIN OF APTEROUS. 



The mutant called ''apterous" was first found by Miss Wallace in 

 the white miniature stock in August 1913. This form continued to 

 appear as occasional individuals, both males and females, for some 

 months, from which it was concluded that the mutant was an autosomal 

 recessive for which only a few white miniature flies were heterozygous. 

 Several attempts to breed apterous individuals to one another or to 

 other flies failed, and it was thought that the form was completely sterile. 



DESCRIPTION OF APTEROUS. 

 The most striking feature of this mutant is, as its name implies, the 

 total absence of wings, the vestiges being in most cases a mere rough- 

 ness. The balancers also are reduced in the same manner as the wings, 

 a condition that was first found in the case of vestigial. The apterous 

 flies are small in size, rather pale in color, and markedly sluggish in 

 movement; they easily become entangled in food or cotton and drown 

 or dry up. Even when kept very carefully under the best conditions 

 they seldom live more than three or four days. 



INHERITANCE OF APTEROUS. 



At this stage Metz (C. W. Metz, Am. Nat. 1914, pp. 675-692) began 

 work with the mutation and found that the failure to breed was largely 

 because the males were too weak to copulate and the females pro- 

 duced few or only rudimentary eggs. In only three cases out of more 

 than a hundred did apterous females give offspring when crossed to 

 normal males, and there was correspondingly only a single case of 

 fertilization by an apterous male. From these crosses and from 

 inbreeding other pairs heterozygous for apterous there were produced 

 a total of 1,405 wild-type to 450 apterous flies, or 24.3 per cent apter- 

 ous, which is a remarkably close approach to expectation, in spite of 

 the weakness of the apterous flies. 



