392 H. G. MAY. 



If old plugs were used over again they were either sterilized and 

 kept in closed fruit jars or they were kept in closed jars for at 

 least a week before using. Only plugs that had not been con- 

 taminated with food were used again. 



But the evidence against the theory of contamination has 

 little to do with the precautions taken in handling the material. 

 In the first place all flies that appeared in the vestigial-winged 

 lines had vestigial wings and all flies that appeared in the long- 

 winged lines had long wings. The chief reason for choosing the 

 vestigial-winged race for the selections was the fact that the 

 second recessive could be used as a check in case of contamination. 

 In all of the experiments no long-winged fly appeared in the 

 vestigial-winged lines and no vestigial-winged form appeared in 

 the long-winged lines. If the full-eyed and heterozygous flies 

 had been due to contamination then the other characters should 

 also have appeared, especially since for a long time the breeding 

 vials of the two races were intermingled and treated as one lot. 

 In the second place the author was handling no full-eyed, vesti- 

 gial-winged flies at the time the first male appeared. It is true 

 that Professor Zeleny had his stock of such flies in the same 

 room at the time, but they were kept on a table about twenty 

 feet from the one used for this work and the chances that a 

 vestigial-winged fly will travel that distance are not very great. 

 Finally, if the flies were due to contamination, full-eyed females 

 as well as full-eyed males should have appeared; indeed, full- 

 eyed females should have been more frequent than heterozygous 

 females, but all females were heterozygous. 



It is interesting to note that in all cases except one where full- 

 eyed or heterozygous flies appeared in the selected lines it was 

 in the high lines. The one exception w r as the heterozygous 

 female in h3f2, a reverse selection from a low line. 



More significant, however, is the fact that the females were 

 always heterozygous. That means that if the change took 

 place after fertilization only one chromosome in case of the 

 females was affected. The males, of course, have only one 

 chromosome bearing the factor. If the change occurred before 

 fertilization, then it is probable that it appeared in the female 

 germ cells. Had it taken place also in the male germ cells then 



