436 GEO. T. HARGITT AND WALTER W. FRAY 



reduced to a minimum. It was believed that Paramecium swim- 

 ming about in the hquid contained in the watch crystal would 

 soon free itself of bacteria, and that the large amount of liquid 

 present would so dilute the bacteria as to reduce the number of 

 washings necessary. However a single Paramecium aurelia 

 swimming in a large volume of liquid in a watch crystal was 

 often lost or else a great deal of time was consumed in search- 

 ing through the liquid and recovering it. This method was 

 therefore, abandoned. 



In the final attempt only small amounts of the sterile medium 

 were used for washing and this was done in the ordinary depres- 

 sion slides enclosed in Petri-dishes. The animal was isolated 

 with a sterile pipette, the cover of the dish raised to permit the 

 entrance of the capillary tube, Paramecium was deposited in the 

 sterile medium of the slide, and the cover of the dish dropped. 

 Paramecium was washed in this way (using a fresh sterile pipette 

 each time) through five different fluids. To determine the reduc- 

 tion in number of bacteria by this washing and whether sterili- 

 zation was accomplished, each wash fluid was examined bac- 

 teriologically. Agar plates were inoculated with a little more of 

 the wash water than would ordinarily be carried over in the 

 pipette when making a change from one slide to another. In the 

 last plate (in these tests) Paramecium itself was introduced to- 

 gether with the wash fluid, so that we might determine whether 

 there were any bacteria still sticking to the animal. After a 

 few days incubation the colonies which developed on the plates 

 were counted. The number of bacteria in each wash fluid is 

 indicated in the following table, which includes the washing in 

 the watch crystals and in the depression slides. 



This table shows the advantage of washing Paramecium in 

 depression slides enclosed in Petri-dishes. The animals are at 

 least approximately sterile after the third washing and whatever 

 contaminating bacteria may still be present would certainly be 

 removed by the fourth or fifth washing. The fact that the fifth 

 plate showed no developing bacteria is proof that Paramecium 

 was entirely free from bacteria, for the animal was included in 

 the water that was placed in the fifth plate. Had any bacteria 



