80 "PURE MIXED CULTURES 11 OF AM(EBA 



suitable concentration is obtained. This method 

 is quite simple, and with a little practice one can 

 make half a dozen cultures (each from a single cyst) 

 within an hour. The cysts are easily recognised 

 with a low power, and to make sure that only 

 one is present the whole field can be traversed with 

 a high power. In the same way individual living 

 amcebae can be planted out, but are not always 

 distinguishable from debris, etc., and one might 

 overlook the presence of a very small form. We 

 have also by this means isolated various flagellates, 

 fungi, etc., from other organisms which might 

 lead to confusion in impure cultures. The 

 necessity for separating a single amoeba was made 

 still more evident to us by the appearance, in 

 some of the cultures, of an organism (fig. 25) 

 which was at first mistaken for the amoeba, 

 and which gave us considerable trouble for some 

 time until it was removed. When showing this 

 method to others we have taken the opportunity 

 to make repeated subcultures from the progeny 

 of a single cyst, so that the risk of con- 

 tamination by other amoebae is reduced to a 

 minimum. 



But although these methods enable single 

 amcebae or cysts to be separated from their fellows, 

 they do not supply us with the individuals free 

 from bacteria. 



2. PREPARATION OF AMOSBJE WITH PURE 

 BACTERIAL STRAINS 



Preliminary Remarks. An impure culture of 

 amoeba will as a rule be found to contain bacteria 



