ENCYSTMENT 47 



chemical point of view, it having been usually 

 attributed to such agencies as drought, lack of 

 food, etc., the amoeba being endowed with a sort 

 of intelligence which enables it to defend itself 

 against these injurious conditions. Our conten- 

 tion at the outset was that the amoeba is actually 

 compelled to protect itself by the action of soluble 

 substances in the environment, which are of the 

 nature of poisons, and which are produced under 

 natural conditions by those bacteria that are in- 

 variably associated with the growth of amoebae. 



After confinement for a variable length of time 

 within the walls of the cyst the amoeba is enabled 

 to escape, and the series of changes which take 

 place during this process are known as " excysta- 

 tion." On a priori grounds, and from our previous 

 experience with Polytoma (fig. 18), we were in- 

 clined to think that the causes of these changes 

 must also be looked for among bacterial products, 

 which act on the cyst walls from without, and 

 that the possible influence of any causes of ex- 

 cystation acting from within the cyst were only 

 of a secondary nature. The experiments described 

 in this chapter indicate the preliminary steps 

 undertaken to elucidate these problems. 



The Cysts. The typical "dauer" cysts in this 

 species are spherical refractile bodies about 10-12 p 

 in diameter (fig. 19). In cultures on jelly media, 

 however, their shape is liable to be modified by 

 mutual pressure, and they may be kidney-shaped or 

 otherwise irregular. Their size also varies consider- 

 ably, and many may be twice or even four times as 

 large as others. They may be described as consist- 

 ing of a central dense mass of protoplasm, containing 



