10 



J. PERCY BAUMBERGER 



Treatments of five minutes seldom kill the pupae, and in 90 per 

 cent of the cases render them sterile. The sterilizing effect was 

 not entirely understood till pupae were used which came from a 

 Drosophila stock bottle of fermenting banana contaminated by 

 molds and bacteria. These pupae when washed with 85 per 

 cent alcohol saturated with HgCL were steriUzed in less than 50 

 per cent of the cases as shown in table 2. 



This indicates that the sterilization involves two stages, 1) de- 

 struction of molds and bacteria by feeding of the larvae and a 

 good strong yeast growth and, 2) killing of yeast by alcohol. 



The toxicity of alcohol for yeast cells is shown to be high by the 

 following experiments : 



Three grams of yeast were separated in 25 cc. of sterile water 

 and two drops of this fluid were added to each of ten watch-glasses 

 filled with 85 per cent alcohol and to ten sterile banana-agar tubes. 



TABLE 2 



After 1, 5, 10, 15, and 20 minutes, respectively, two sterile banana 

 agar tubes were inoculated with two drops of yeast from the 

 watch-glasses of alcohol. The tubes were kept under observation 

 for 21 days. The results in table 3 show that a five-minute ex- 

 posure to 85 per cent alcohol is fatal to yeast cells. ^ 



h. Test of sterility. The sterility of a culture tube could usu- 

 ally be judged by the fact that no growth occurred, 1) around the 

 pupae which were placed on the medium, 2) at spots where adults 

 regurgitated on the medium, 3) at adult fecal spots, 4) upon ovi- 

 position, 5) upon emergence of larvae, 6) upon pupation of larvae. 

 If no growth occurred in the first case, i.e., around the pupae, the 

 medium showed no sign of contamination throughout the life- 

 cycle. Bacterial growths visible around the pupae might dis- 

 appear during the life of the larvae, but usually reappeared when 



" Paine ('11) showed that yeast cells are highl}' permeable to alcohol which 

 readily and permanently plasmolyzes them. 



