ZOOLOGY: J. P. BAUMBERGER 
123 
invariably carried yeast cells and often had a rod-like bacterium upon 
them. This made it appear that an internal symbiotic relationship 
existed comparable to the notable cases of the flesh fly ,2 in which a 
micrococcus is passed through the egg and aids the larvae in the diges- 
tion of albumins, etc., the cockroach,^ which has certain cefls of the 
fat-body reserved for yeast, and many plant bugs,^ which have caeca 
of the alimentary tract containing certain bacteria. 
In the following experiments undertaken to show that microorgan- 
isms are not transmitted through the egg of Drosophila, the first pre- 
caution was to free the insect from external microorganisms. Usually 
eggs were used for this purpose but their small size makes this a diffi- 
cult procedure. As it is well known that the lining of the digestive 
tract of larvae is thrown off upon pupation, pupae were selected for 
sterilization. 
(1) The pupae were submerged in 85% alcohol for ten minutes and 
then introduced aseptically 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 loopfuls of 
the medium into a sterile tube of similar food. It had previously been 
determined that yeast developed readily on fermented banana agar. 
This is probably due to the catalytic action of dead yeast,^ and the abun- 
dance of nutrition supplied by the banana.® 
(2) Larvae which had been feeding on media containing living yeast 
ceils were submerged and washed in 85% alcohol and then introduced 
into sterile culture tubes. In all cases yeast developed on the new 
media. Cultures from the digestive tracts of the larvae gave similar 
results. Apparently many cells escape digestion in the stomach as 
is the case with seeds or insect eggs in birds. 
(3) Eggs were sterilized by soaking in 85% alcohol for ten minutes. 
The larvae which hatched were always aseptic. 
From the foregoing experiments we may conclude that yeast is not 
present in the eggs or pupae of Drosophila. However, a loose symbiosis 
exists between the two organisms. As mentioned above, surface fun- 
gus growths disappear in the presence of larvae which often seemed to 
be more numerous at this point. From these observations I inferred 
that the larvae fed upon the microorganisms present. 
The problem was attacked by the use of several different media. 
Two dozen large ripe bananas were fermented overnight by adding 24 
grams of compressed bread yeast separated in a few cubic centimeters 
