148 RUTH L. PHILLIPS 



the animals more frequently than every few weeks, the only pre- 

 cautions taken against contamination with foreign bacteria in 

 the daily transfer were those which insured sterile glassware and 

 sterile media and avoidance of air-borne bacteria during trans- 

 fer. This procedure enabled me to keep accurate daily records 

 of at times as many as eighty cultures without infringing upon the 

 routine of other work. 



Careful record of the rate of division was kept, and each death 

 noted, as it was considered important to have a record of the 

 death rate as well as the rate of fission in determining the effect 

 of food upon the animals. Previous work of Woodruff ('11) has 

 established the fact that one may disregard the effect of light, 

 ordinary ranges of temperature, and barometric pressure in such 

 experimental work as this. It was, therefore, not considered 

 necessary to keep records of these features. 



A series of tables was compiled from the daily records by 

 averaging for three-day intervals. The very minor fluctuations 

 in division rate which appear in the daily observations, and which 

 depend so much upon the personal equation of the observer, 

 disappear when the three-day intervals are used ; but the five-day 

 interval, which was also tried, causes too much removal of small 

 differences in rate of fission for a thorough study of changes in 

 metabolism due to the influence of food. The tables show the 

 average number of divisions for the time covered by a given set 

 of observations, the percentage of high divisions, the percentage 

 of low divisions, and the percentage of deaths. These percent- 

 ages were found to be helpful in the interpretation of results. 

 The graphs were made from tables in which the averages were 

 computed for the three-day intervals. 



OBSERVATIONS 



a. Growth of Paramecium aurelia in certain pure cultures of 

 bacteria in 0.1 per cent standard tiinothy hay infusion 



Nine pure cultures of bacteria and twelve mixtures of these 

 were fed at different times to pure lines of Paramecium. These 

 bacteria have already been described as to cultural characteris- 



