90 "PURE MIXED CULTURES" OF AMCEBA 



might pass through them and leave the mass of 

 mixed bacteria behind. On one occasion an amo3ba 

 was found beyond the second wall, but it could not 

 be recovered. When the amoeba reached the first 

 wall, they either encysted or degenerated, and 

 formed a solid-looking mass. Having noticed that 

 amoebae crawl along the hyphas of moulds growing 

 in the medium, fine hairs and glass tubes were set 

 radiating from the centre, but water collects round 

 them and there are more bacteria there than at other 

 parts. With fine radiating grooves in the jelly no 

 particular advantage could be seen. Lastly, amoebae 

 are said to have been isolated by the principle of 

 geotropism, and by electrical methods (Mouton). 



(3) Heat. The possibility of this means being 

 useful to destroy bacteria without injuring the 

 cysts was suggested by a reference to an article by 

 Musgrave and Clegg, in which the method was 

 mentioned as worthy of a trial. This appealed to 

 us, as we thought that by employing some method 

 of intermittent sterilisation at say 65 C. it might 

 be possible to remove even sporing bacteria. Old 

 cysts were used, as they were expected to be more 

 resistant than young ones. A number of cultures 

 were exposed respectively to temperatures ranging 

 from 60 C. to 80 C. for half an hour on four 

 consecutive days. No amoebae could be recovered 

 from any of the cysts, the contents of which in fact 

 looked disorganised. We have, since this, more 

 accurately determined the thermal death-point of 

 amoebae cysts, and find that, although some will 

 stand 58 C. for half an hour, none of them will 

 live at 60 C. As most water bacteria require 

 at least 55 C. to destroy them, we think that the 



