438 GEO. T. HAEGITT AND WALTER W. FRAY 



made the rest of the process could be finished in five minutes. 

 The depression slides were filled with sterile water, and the 

 protozoa were carried successively through five of them. In 

 introducing the sterile water and in changing Paramecium from 

 one slide to the next the cover of the Petri-dish was lifted just 

 far enough to permit the insertion and withdrawal of the capil- 

 lary portion of the pipette. The chance of bacterial contamina- 

 tion was so slight as to be negligible, being no greater, indeed, 

 than in ordinary bacteriological procedure. 



The next test was that of the moist chambers. It is essential 

 to keep the slide cultures in moist chambers in order to prevent 

 evaporation of the cultiu-e fluid, but how were we to sterilize 

 our moist chambers and especially was there any chance of 

 maintaining a sterile atmosphere within them? If not, it would 

 be impossible to continue the problem since the sUde cultures 

 would soon be contaminated by foreign bacteria and Paramecium 

 would be feeding on unknown, instead of known, organisms. 

 In order to get some idea of the relative numbers of bacteria 

 in a moist chamber under ordinary unsterilized conditions as 

 compared with the number of bacteria in the air of the labora- 

 tory, two sterile agar plates were prepared; one was left exposed 

 to the air of the covered moist chamber, the other to the air of 

 the room for two hours. The number of colonies on the plates 

 were counted after two days growth and showed there was but 

 one-tenth the chance of contamination in the chamber as com- 

 pared with the air of the laboratory. But the plate within the 

 moist chamber had some colonies of bacteria and the chance of 

 contamination of the cultures kept within the chamber was still 

 too great. If such a study is to have any significance the great- 

 est possible pains must be taken to avoid any contamination, if 

 such a thing is possible. 



The next plan was to use Petri-dishes as moist ' chambers. 

 The dishes used were just large enough to contain a single sUde. 

 These dishes with their enclosed depression slides were sterilized 

 in the hot air sterihzer and were kept closed till ready to be 

 used. Then with a capillary pipette the depression was filled with 

 the culture fluid containing the strain of bacteria to be used. 



