EXCYSTATION 75 



tube 0'1-per-cent. tyrosin and 0'1-per-cent. choline 

 hydrochloride were added, and the surface smeared 

 with sterile horse serum. Cysts with only a few 

 accompanying dead bacteria were placed on this 

 and allowed to grow for a week, when the living 

 amoeba? were transferred to a second slope made in 

 exactly the same way. This subculture was again 

 removed after a week to a third similar slope and 

 the amoebae on examination were found to look 

 fairly healthy, although rather pale and transparent. 

 No bacteria, living or dead, could be distinguished 

 and the cultures were sterile when tested in the 

 usual way. On the third day the culture became 

 contaminated, upon which the amcebaB rapidly 

 encysted. Simultaneously with this experiment, a 

 similar tube was prepared, but in this case the 

 " chloroformed solution " already described was 

 employed instead of horse serum, and although 

 perhaps not quite so effective we were able to 

 maintain the culture for about a fortnight. It is 

 thus possible to grow amoebae for a short time even 

 in the absence of dead bacteria, but the growth is 

 never so abundant as when living bacteria are 

 present, and this could hardly be expected under 

 the conditions of artificial feeding and induced re- 

 production to which they have been subjected. 



SUMMARY AND CONCLUSIONS 



Excystation. Amoeba cysts which have been 

 freed from accompanying bacteria (living and dead), 

 and from soluble bacterial products, do not produce 

 free amoeba* when placed in pure water. Excysta- 

 tion will, however, take place in the presence either 



