440 GEO. T. HARGITT AND WALTER W. FRAY 



Another control was carried on as a check on the accuracy of 

 results and methods. This concerns the determination of the 

 extent to which pure cultures of bacteria in the slide cultures 

 remain uncontaminated for a long period. In every transfer 

 of Paramecium to fresh medium, or in any other manipulation 

 in the Petri-dish moist chamber, sterile pipettes were always 

 used. A pipette once used for one purpose was not used again 

 for that or any other purpose till it had undergone a steriliza- 

 tion in a closed vessel in the hot air sterilizer for a half hour at 

 170°C. Doubtless this had much to do with the reducing of 

 chances for contamination by bacteria. After Paramecium had 

 been grown in pure cultures of bacteria for 10 days or more, 

 having been transferred to fresh media each day, the last medium 

 wa.s plated on an agar plate, also some of the water in the bottom 

 of the dish was plated. In one such case it was found that 

 only one contaminating colony was on the plate to 350 other 

 colonies of the pure culture used. In other cases, which, repre- 

 sent the usual condition, there were no strange bacteria present. 

 Thus at the end of this period in the most extreme cases only 

 one out of 350 bacteria was a strange one; one contaminating 

 bacterium outnumbered in this way can not be considered as 

 seriously modifying the food of the Paramecium. There was no 

 possible question as to whether contamination occurred or not. 

 The morphological and cultural characteristics of the pure cul- 

 ture of bacteria used were well known; in the test any colony 

 with different method of growth, different color, or other differ- 

 ence would at once be noted. If bacteria of the same sort gained 

 entrance from the air it could not be detected, but in such a case 

 there would, of course, be no contamination, i.e., no introduc- 

 tion of a different kind of bacteria. 



Under the discussion of the results of growth of Paramecium 

 it will later be noted that from time to time certain of the pro- 

 tozoa were transferred to a new medium or to a new kind of 

 bacteria. In every such case there was first a washing clean 

 of the Paramecium before the transfer was made, and . later 

 tests would show that none of the old bacteria had been trans- 

 ferred. 



