6 J. PERCY BAUMBERGER 



inent, while the posterior end is drawn up. In more fibrous ma- 

 terial, the mouthparts probably aid the larva in moving about. 

 When fully grown, it leaves the medium to pupate on the side of 

 the test-tube or the surface of the medium itself. 



d. Ecology of cultures. Drosophila is very extensively^ used by 

 geneticists in breeding experiments. The insect is reared in small 

 glass bottles or milk jars, plugged with cotton and containing fer- 

 menting banana covered with absorbent paper. Quite often 

 these 'cultures go bad,' i.e., smell strongly of acetic acid or be- 

 come putrid or covered with mold, so that the insects are de- 

 stroyed and the breeding experiment terminated. 



The method commonly employed in making the culture media 

 is to boil skinned bananas, to cool the mass and to add two cakes 

 (24 grams) of Fleishmann's bread yeast (bottom yeast) per dozen 

 bananas. This is allowed to ferment and is used as a stock sup- 

 ply from which to prepare clean culture bottles. In this manner 

 the medium is kept fairly sweet, probably due to the great develop- 

 ment of the yeast, with an accompanying production of alcohol 

 which retards^ the development of molds and bacteria. 



If pupae are taken from a bottle that has gone 'bad' and placed 

 on banana agar, a number of different bacteria or molds may 

 develop around then, prominent among which are a mucor, 

 Rhizopus nigricans Ehrenberg, the bread mold, Aspergillus, 

 the green herbarium mold, Penicillium glaucum, the blue mold, 

 and the acetic acid bacillus. If pupae are taken from a good 

 culture tube with yeast alone or yeast and the acetic acid 



5 In this connection Lafar ('10, II, 2, pp. 238-240) writes: "From the stand- 

 point of the oecological theory of fermentation, the alcohol produced by yeast 

 should be regarded as a weapon capable of hindering the appearance of other fun- 

 goid competitors in saccharine nutrient media. However, Avhen accumulated in 

 the medium during the progress of fermentation, it also restricts the further de- 

 velopment and action of its producer. In this case, as with yeast poisons in gen- 

 eral, the first result is the cessation of cell reproduction, a larger quantity of alco- 

 hol being necessary to arrest fermentation and a still further quantity to kill the 

 cells." Reproduction of yeast cells ceases at a G per cent and fermentation at a 

 5 to 24 per cent concentration of alcohol. It should be also remembered that 

 most bacterial or fungus cultures have a tendency to become pure, probably owing 

 to the production of some definite antagonistic substance, or to better adaptation 

 to the medium by the successful form (Hiss and Zinsser, '10). 



