14 J. PERCY BAUMBERGER 



The nutrient medium for yeast in itself is not an adequate sub- 

 stitute for sterile fruit, as Drosophila larvae live longer on the 

 latter, e.g., 



Medium Increase in Length Longevity 



Sterile banana agar 1.8 mm. 26 to 44 days 



Sterile yeast nutrient medium mm. 5 days 



Therefore sterile fruit has greater food value for sterile larvae 

 then the simplest 'nutrient medium for yeast.' Fruit is mainly 

 the nutrient substratum for yeast cells, but has some food value 

 for Drosophila larvae. 



c. Are products of fermentation essential food requirements 

 of Drosophila larvae? In the preceding experiments living 

 yeast cells had an opportunity to develop and form products of 

 fermentation in the media. As these products may have food 

 value for the larvae, the essential difference between a septic and 

 a sterile food might be the absence of these substances. If this 

 were the case, the larvae would be dependent on yeast not as a 

 food, but as a chemical agent. 



By boiling yeast before adding it to yeast nutrient agar, the 

 formation of fermentation by-products was prevented. Fleish- 

 mann's bread yeast was used for this purpose and 6 grams were 

 added to every 100 cc. of yeast nutrient agar. On this medium 

 sterile larvae grew at a normal rate, reaching their full size in ten 

 days and pupating normally. This proves that Drosophila lar- 

 vae grow normally on dead yeast in the absence of any by-products 

 of fermentation. 



d. Is yeast a complete food for Drosophila larvae? In the 

 media used thus far various substances besides yeast were present. 

 To eliminate these and determine whether or not yeast alone is a 

 complete food for Drosophila larvae, media were made up of 

 Fleishmann's compressed bread yeast, water, and agar-agar. ^^^ 

 It was found that sterile larvae on a medium of 6 grams yeast 

 per 100 cc. water grew as rapidly as non-sterile lar\^ae and many 

 times faster than sterile larvae on banana. In figure 3 cultures 

 W 3, 4, and 5 show that larvae on dead yeast grew to maximum 



1° Sterile larvae live a maximum of five days on sterile 1| per cent agar and 

 water medium, showing no increase in size. 



