1924] Kessel: Amoebae of Culture Rats and Mice 503 



Precystic stage. — In the motile stages of this amoeba, a certain 

 number of the vacuoles usually contain solid food material. As the 

 amoeba prepares for eneystment, all solid material is either digested 

 or egested, leaving the vacuoles filled with liquid substance only. 

 These vacuoles tend to coalesce, and a few large vacuoles result from 

 a number of small ones (pi. 43, fig. 5). 



The amoeba assumes a spherical form and in time secretes a cyst 

 wall (pi. 43, fig. 7). There is a further coalescing of the vacuoles at 

 about this stage (fig. 6), until a single large vacuole is formed in the 

 early phase of eneystment (fig. 7). This vacuole has the subcentral 

 position of the glycogen mass, the glycogen having been dissolved out 

 in the process of staining. It seems certain, however, that this gly- 

 cogen material is derived, at least in part, from the smaller vacuoles 

 found in the precystic stages that contain liquid. There is an analogy 

 between this method of formation of vacuoles and the method of the 

 formation of contractile vacuoles in ciliates as described by Taylor 

 (1923). 



Encystment 



Occurrence. — Until recently it was generally considered that 

 encystment was for the purpose of protecting the organism while it 

 was being transferred from host to host. There seems, however, to be 

 growing evidence that encystment may occur periodically, following 

 intervals of the accumulation of excess food material (Kofoid, 1923). 

 It has been noted that, in ca.ses where the cysts appear in the faecal 

 material, they also usually occur in the caecum, indicating that encyst- 

 ment, for the most part, takes place in the caecum. It is further signi- 

 ficant to note that following the administration of ep.som salt there 

 is no regular response in the formation of cysts. One would think 

 that, as a result of exposure to this deleterious environment, the forms 

 would encyst immediately. This does happen occasionally and a few 

 hours after the extrusion of the amoebae has begun, cysts may be 

 found in the faeces. It happens just as often, however, that no cysts 

 appear in the faeces, although motile amoebae may appear for a period 

 of two or three days or until the final effects of the purgative have dis- 

 appeared. Numerous cysts are often encountered on the first examin- 

 ation after the purge is administered, indicating that the cysts were 

 already formed in the caecum, and that the caecal contents, whether 

 they contain free amoebae, encysted forms, or a mixture of the two, 

 are emptied to the outside. 



