PARAMECIUM IN PURE CULTURES OF BACTERIA 431 



the work was later limited to P. aurelia. The results of both 

 are given but those of P. aurelia cover a longer period of time. 

 For a discussion of the differences of the two species the reader 

 is referred to Jennings and Hargitt ('10), and Woodruff ('11 b). 

 In the starting of the pure strains of the Paramecium the single 

 individuals were isolated in the usual way with capillary pipettes 

 and grown in depression shdes, or in dishes to produce large 

 cidtures. 



Before starting the experiments on growth in pure cultures 

 of bacteria the protozoa were grown in depression slides for a 

 time in the usual manner, depending upon chance inoculation 

 of the media to furnish the bacterial food. The purpose of this 

 was to deternnne the normal rate of fission, to acclimatize the 

 animals to growth in a limited amount of medium, if such was 

 necessary, and to determine what was the best medium for this 

 growth. The slides were used without cover glasses and were 

 kept in moist chambers to prevent evaporation of the fluid, the 

 usual practice for work of this kind. In the initial experiment 

 the medium used was hay infusion liquid taken from a stock 

 culture and filtered to remove the protozoa. This medium was 

 placed in the depression slides and examined with a binocular 

 microscope to be certain that no strange paramecia, and no 

 protozoa of any kind, were present. 



The rates of fission for two weeks averaged approximately one 

 per day for P. caudatum and nearly one and a half for P. aurelia. 

 As might have been expected this filtered infusion was found 

 not to be entirely satisfactoiy. It varied in composition and in 

 age and was sometimes too rich, since the bacteria increased so 

 rapidly as to hamper the activities of Paramecium. The stand- 

 ard 1 per cent hay infusion was also unfavorable if used full 

 strength, and it was necessary to dilute it somewhat. It was 

 obvious that if hay infusion was to be used as a culture medium 

 it must be so standardized in its preparation that its composition 

 did not vary, and also it must be of a concentration suitable for 

 use in the depression slides. To determine what concentration 

 was best the standard infusion was diluted 20 times, 10 times, 

 and 5 times with water making standard infusions of 0.05 per 



