446 GEO. T. HARGITT AND WALTER W. FRAY 



might stimulate Paramecium somewhat in its growth and fission- 

 rate, but almost at once the divisions dropped below normal, 

 ceased, and death of the protozoan took place. 



Bacterium XI is Bacillus subtilis, present on hay and in the 

 air and is so characteristic an organism in hay infusions as to 

 have received the name of hay-infusion bacillus. This is the 

 only one of the bacteria which appeared to be at all favorable 

 as a single source of food. During the period of two weeks 

 recorded in table 7, Paramecium had a higher rate of division 

 in pure cultures of this organism than in the control cul- 

 ture of mixed bacteria. The rate of division was nearly as high, 

 but not quite, as the division rate in control cultures at any 

 time during the two months in which these cultures were carried 

 on. This suggests that Bacillus subtilis is nearly as favorable a 

 food as the usual mixed flora found in all hay infusions; or the 

 conclusion might be drawn that B. subtilis in the ordinary infu- 

 sion is the chief dependence of Paramecium for food and the 

 other organisms play little part in a nutritional role. To test 

 the matter a little further and to find out the possible effect of 

 a change of diet without a change of medium the following cul- 

 tures were run. At one of the division periods of the para- 

 mecia in the pure culture of B. subtilis, one of the cells was 

 transferred to beef extract of 0.025 per cent in which there was 

 a pure culture of the same bacillus. Another one of the cells 

 from the same source was retained in the 0.1 per cent hay infu- 

 sion but the bacteria were of a mixed and unknown sort, such 

 as are found in all normal hay infusions. In table 7 these new 

 cultures are the last two of the lot and are headed ''Bact. XI, 

 transferred to beef broth/' and ''Bact. XI, transferred to mixed 

 culture." In the mixed culture Paramecium continued to repro- 

 duce at about the same rate, or a little faster, while the Para- 

 mecium transferred to the beef extract but with precisely the 

 same food, lowered its rate of division and for 8 days had an 

 average rate of about 0.75 per day. One or two experiments 

 are not sufficient to warrant anything definite, but it appears 

 as though the change of medium had a depressing effect, while 



