52 ENCYSTMENT AND EXCYSTATION OF AMCEBA 



on a plain 2-per-cent. agar jelly made faintly acid 

 with citric acid, and covered with a shallow layer 

 of distilled water containing a little litmus which 

 turned red. The acid water was poured off daily 

 and the cysts examined. Free amoebae were ob- 

 served on the fourth day, bacteria growing very 

 slowly, and the medium remaining acid through- 

 out. Sluggishly moving amoebae and cysts were 

 present together for about a week, the relative 

 proportion altering very little, whereas under 

 normal conditions one usually finds a rapid increase 

 in the number of cysts in the same period. After 

 ten days the preparation was allowed to concen- 

 trate slowly, and it was noticed that the amoebae 

 did not encyst, but simply became more or less 

 rounded and motionless (fig. 23). This resting 

 condition without encystment has also been pro- 

 duced on alkaline media, where the alkalinity 

 reduced bacterial growth to a minimum. The 

 appearance of these quiescent amoebae is quite 

 distinct from that of cysts, as they have no definite 

 wall and their shape is more irregular. They 

 cannot be mistaken for dead amoebae, which are 

 usually quite spherical. They are probably forms 

 which are unable to encyst as the bacterial pro- 

 ducts to which they are exposed are too dilute to 

 effect this. 



Influence of Bacterial Products. The next ex- 

 periments were undertaken to obtain more positive 

 evidence as to what the conditions are which cause 

 encystment, and if possible their mode of action. 

 The first point noticed was that amoebae are 

 influenced greatly in this respect by the amount 

 of bacterial growth present: for instance, those 



