EXCYSTATION 66 



products of growth of S. albus were less effective 

 on a live slide than on a jelly medium. A similar 

 experiment carried out with a mixture of the 

 common bacteria found in tap water gave the same 

 result, excystation taking place in response to some 

 stimulus from the soluble products of bacterial 

 growth. 



While the preceding experiments were in pro- 

 gress, we had been endeavouring to find a more 

 reliable means of separating the amoeba cysts from 

 living bacteria, which would be available as a 

 routine process, and having by this time been able 

 to accomplish this, and to obtain cultures of 

 amcebae with pure strains of bacteria, we were in 

 a better position to test the working hypothesis, 

 i.e. that excystation is due to bacterial products 

 on which the former experiments were based. The 

 details of the stages in the separation of amoebae 

 from living bacteria were of a complicated nature, 

 and they are, for convenience, described later in 

 a separate chapter (Chap. III.). It is only neces- 

 sary to point out that, from this time onwards, all 

 the experiments were carried out with amcebae 

 which were in culture either with a pure strain of 

 a bacterium or with no living bacteria at all. 



The effect of gelatin liquefying and non-liquefying 

 bacteria on excystation. While trying the effect of 

 gelatin media on the growth of amcebae in impure 

 cultures it occurred to us that by employing pure 

 cultures with bacteria which liquefy gelatin and 

 with those which do not, some difference in the 

 rate of excystation might be made out. With 

 this object a series of pure mixed cultures with a 

 variety of non-liquefying bacteria was made, em- 

 iv 5 



