50 ENCYSTMENT AND EXCYSTATION OF AM(EBA 



hand, amoebae do not stand rapid drying; they 

 maintain their appearance unchanged so long as 

 there is the least possible quantity of moisture 

 adhering to them, and then suddenly assume a 

 globular shape and rapidly die. We had already 

 formed the opinion that it is probably the con- 

 centration of injurious bacterial products in solu- 

 tion during drought which is the primary cause of 

 encystment, and it will be shown that amcebse can 

 be made to encyst on a live slide by means of such 

 a substance as choline. To determine if encyst- 

 ment will occur on gradual drying in the absence 

 of such bacterial products, cultures were made by 

 the moist-chamber method, and when sufficient 

 amoebae had wandered from the site of inoculation, 

 the Petri dishes were exposed to the air, so that 

 the jelly film could slowly dry up. It was then 

 found that only those amoebae near the centre of 

 the growth encysted, while those far distant, which 

 were comparatively bacteria free, persisted in the 

 free state until the whole of the jelly had dried up, 

 when they rounded up and burst, liberating their 

 nuclei, which remained visible for several days. 

 Therefore encystment is evidently not due to 

 gradual drying. It may be pointed out that 

 amoebae can also encyst when water is abundant 

 and bacterial products are apparently dilute, but 

 we think this is explicable by the fact that the 

 condition of the medium in which an amoeba is 

 found is no indication of the nature of the im- 

 mediate environment of an individual, since the 

 bottom layers of a solution which contains amoebae 

 and multiplying bacteria are not measurable by the 

 average condition of the whole medium. Again, 



