PARAMECIUM IN PURE CULTURES OF BACTERIA 447 



the change of food had a slightly accelerating or stimulating 

 effect. 



There followed a period of several weeks during which the 

 precautions with regard to contamination from air bacteria were 

 relaxed, and the slide cultures became inoculated with various 

 bacteria by air infection. The sUde culture originally in pure 

 Bacillus subtihs (Bact. XI) was one of these and for these several 

 weeks it was permitted to grow in this mixed culture and had a 

 rate of division of a little over one per day. Beginning May 25 

 one of the animals of this line was again carefully observed and 

 its divisions recorded. Within a few days, at division periods, 

 two other lines were started from this one, by sterilizing the 

 protozoa and placing them in pure cultures of bacteria. One 

 of these lines was returned to the same food it originally had, 

 B. subtihs (Bact. XI), while the other was fed upon Bacillus 

 fluorescens (Bact. III). We thus have three lines derived from 

 the original B. subtilis line all feeding upon different bacteria. 

 The number of divisions noted in these lines in the three weeks 

 following are tabulated in table 8. 



The table shows the rate of division of Paramecium in the 

 cultures of mixed bacteria to be 1.143 per day if averaged for 

 the entire three weeks of the experiment, or 1.2 for the first 

 two weeks and 1.0 per day for the last week. Two other para- 

 mecia of the same pure line in previous experiments averaged 

 1.749 divisions per day for 12 days, and 1.143 per day for two 

 weeks, respectively. All of these hues if averaged would give 

 1.345 divisions per day as a general average for Paramecium 

 aurelia in cultures of mixed bacteria. The original line grown 

 in pure cultures of Bacillus subtihs divided, on the average, 

 1.214 times per day while for the two weeks recorded here the 

 rate was only 0.6 per day, only about one half as great. This 

 means, then, that the paramecia which had originally fed on B. 

 subtilis continued to do as well and divided as often when placed 

 on a diet of mixed bacteria for three weeks, but fell off greatly 

 when again f^d on B. subtihs. Progeny of this same hne fed on 

 Bacillus fluorescens did even more poorly, the average rate of 



