ENCYSTMENT 53 



amoebae usually encyst first which are surrounded 

 by dense masses of bacteria. We have been 

 asked frequently how long the free amoebae live 

 before they encyst again. This appears to depend 

 entirely on the dose of bacterial products, such as 

 choline, etc., to which they are subjected. Under 

 certain conditions, such as starvation and drying, 

 they will not encyst at all, as already pointed out ; 

 and recently, by means of cultures without living 

 bacteria, amoebae can be compelled to leave their 

 cysts, and grow and multiply for weeks without 

 encysting. On the other hand, living amcebae 

 placed in old infected cultures are killed outright. 

 In ordinary circumstances the poisoning of the 

 amcebae by bacterial products is a gradual process, 

 and they are thereby allowed sufficient time to 

 encyst. On the live slide already described the 

 greatest difficulty we had to contend with was 

 the inability to make the amcebae encyst, as the 

 slow current beneath the cover-glass removed 

 most of the bacteria and their products from the 

 sphere of action. A little tilting of the slide 

 increased this effect, and the amcebae lived for 

 weeks in the free state. It is, however, possible 

 to make them encyst at will by means of choline, 

 cadaverine, or a mixture of the products of growth 

 of Staphylococcus albus grown in Dunham's peptone 

 water for a week at 37 C. The strength of sub- 

 stances necessary to cause encystment are, as nearly 

 as we can ascertain, choline 1 per cent, (it is more 

 powerful if it is old, containing neurine) and cada- 

 verine between 0*8 per cent, and 1 per cent. The 

 method we use is as follows : amcebae are first 

 grown vigorously on the live slide by means of 

 iv 4* 



