8 J. PERCY BAUMBERGER 



10 grants ashes of yeast 

 10 grams ammonium tartrate 

 100 grams rock candy 

 1000 grams water 



Add agar, sterilize in Arnold sterilizer, slant. 

 Into sterile tubes of these media the introduced adults or pu- 

 pae carry living yeast cells which are distributed through the me- 

 dium by the activity of the larvae. 



/. Are living yeasts present in the egg or pupa? In the follow- 

 ing experiments undertaken to show that microoganisms are not 

 transmitted through the egg of Drosophila, the first precaution 

 was to free the insect from external microorganisms. Usually 

 eggs are used for this purpose, but the small size of Drosophila 

 eggs makes this a difficult procedure. As it is well known that 

 the lining of the digestive tract of larvae is thrown off upon pu- 

 pation, pupae were selected for sterilization. 



The pupae from a culture strong in yeasts were submerged 

 in 85 per cent alcohol for ten minutes and then introduced asep- 

 tically into sterile slant culture tubes of agar-agar and fermented 

 banana filtrate. If no yeast developed around the pupae which 

 were placed on the food, the tube remained sterile after the 

 emergence of adults, oviposition, and hatching of larvae. The 

 sterility of the tube was later tested by introducing a few loop- 

 fuls of the medium into a sterile tube of similar food. It had 

 previously been determined that yeast developed readily on fer- 

 mented banana agar. 



2. Larvae which had been feeding on media containing living 

 yeast cells were submerged and washed in 85 per cent alcohol and 

 then introduced into sterile culture tubes. In all cases yeast de- 

 veloped on the new media. Cultures from the digestive tracts 

 of the larvae gave similar results. Apparently, many cells es- 

 cape digestion in the stomach, as is the case with seeds or insect 

 eggs in birds. 



3. Eggs were sterilized by soaking in 85 per cent alcohol for 

 ten minutes. The larvae which hatched were always sterile. 



From the foregoing experiments we may conclude that living 

 microorganisms are not present in the eggs or pupae of Drosophila. 



