SOIL PROTOZOA 321 



After three days, the protozoa reached the edge of the dish. The proc- 

 ess was repeated until finally a culture of protozoa was obtained free 

 from any bacteria except the species inoculated into the dish. 



A sample of the liquid containing the protozoa may be brought into 

 a fine capillary tube, from which a single drop is ejected on a counting 

 chamber (haemocytometer pattern) and immediately examined under 

 the microscope. When a drop is obtained which contains a single organ- 

 ism, a drop of sterile medium or a culture of the specific bacterium is 

 placed upon a clean cover slip which is then put on the chamber so 

 that both drops coalesce. The chamber is then placed in a Petri dish 

 lined with moist filter paper, and incubated. By projecting the image 

 of the drop upon a screen, the multiplication of the protozoan can be 

 followed. 39 



It is sometimes essential to obtain cultures of protozoa free from living 

 bacteria. This has been done by Peters, 40 who isolated Colpidium 

 colpoda from a hay infusion culture, in a drop of sterile medium, upon 

 a sterile microscope slide. 



By the use of a capillary pipette made of drawn-out glass tubing and fitted 

 with a rubber bulb the organism was transferred, through several changes of 

 sterile medium, upon sterile microscope slides. After about six washings, the 

 organism was transferred to a fresh drop of sterile medium, which was placed in 

 a two inch depression block and covered with a small sterile cover-glass, both of 

 which had been previously sterilized by heat. By a process of trial and error, 

 organisms were finally obtained which divided well in the sterile medium. The 

 culture was then transferred to a sterile tube plugged with cotton. 



Two media were used for the isolation and cultivation of the organism: 



per cent per cent 



I. NaCl 0.06 Glucose 0.03 



KC1 0.0014 Histidine 0.01 



CaCl 2 0.0012 Arginine 0.01 



Na 2 HP0 4 .0001 Ammonium lactate .003 



KH 2 P0 4 0.0001 FeCl 3 Trace 



MgS0 4 0.001 KI Trace 



NaHC0 3 0.002 MnCl 2 Trace 



Phenol red Trace Glass-distilled water 



39 Cutler and Crump, 1923 (p. 318). 



40 Peters, (p. 317). The results need confirmation and should not be con- 

 sidered as conclusive evidence. 



