PARAMECIUM IN PORE CULTURES OF BACTERIA 439 



and with another sterile pipette some sterile water was placed 

 in the dish to furnish the moist atmosphere. The isolated 

 Paramecium, cleaned from all bacteria by washing in the man- 

 ner already described, was then introduced by means of a sterile 

 pipette into the culture medium of the slide. This entire process 

 could be accomphshed more quickly than the description can be 

 written, and during the process the cover of the Petri-dish was 

 not raised more than enough to permit the entrance of the 

 capillary portion of the pipette. The sterilization of the slides 

 and dishes took some time and the change of Paramecium each 

 day to fresh fluid took longer than if a number of sUdes were 

 in a single large moist chamber. But it was possible to place 

 the entire Petri-dish on the stage of a binocular microscope and 

 all observations could be made without opening tne dish. The 

 chance of contaminating bacteria gaining entrance was there- 

 fore reduced to a minimum. Tests showed that, with such 

 precautions, foreign bacteria did not gain entrance. 



A further question demanding an answer was the effect of 

 using a small Petri-dish as a moist chamber as compared with 

 the use of large receptacles of the usual type. To answer this, 

 isolated paramecia in slides, with ordinary hay infusion and its 

 mixed flora of bacteria, were kept in large moist chambers and 

 others with sister paramecia were cultivated in identically the 

 same way except they were placed in Petri-dishes as moist 

 chambers. Such parallel cultures showed absolutely no differ- 

 ence, the rate of division was the same, the variation in rate of 

 division from day to day was the same in the two cases, and 

 there was no greater evaporation of liquid in the small than in 

 the large chambers. Such control cultures were run for several 

 weeks, with some of the sUdes shifted back and forth, and no 

 differences could be detected. It is clear, therefore, that the 

 size of the moist chamber, so far as tested by us, is not a matter 

 of importance. In some respects Petri-dishes make ideal moist 

 chambers, and for certain lines of work would be far better than 

 larger chambers. The Petri-dishes may be piled on top of each 

 other the same as watch glasses and actually require less room 

 than the larger dishes. 



