THE CAUSE AND NATURE OF ENCYSTMENT. 49 



themselves in all instances ceased to show any active forms before 

 they disappeared in the controls. This was probably due to the 

 lack of food. 



Now let us inquire why El and 2 did not encyst at all. Was 

 it because of the removal of the by-products of metabolism? 

 The evidence is very much against such a conclusion. If that 

 were true, we should have expected the cultures of the last 

 mentioned experiment to have encysted before the controls, 

 since in that case metabolic by-products were probably increased. 

 Also, in the case of the cultures seeded with newly excysted 

 individuals in which the medium was replaced with fresh hay 

 infusion every day, we should have expected encystment to have 

 been postponed more than it was, if metabolic by-products were 

 an important factor for encystment. Again in series El and E2 

 active forms remained in the cultures for ten days after filtering 

 was stopped. This, we think, would have been sufficient time 

 for the necessary by-products to accumulate, yet no encystment 

 occurred. The arguments just recounted seem to entirely justify 

 the conclusion that the filtering of a culture and replacing its 

 medium with fresh hay infusion defers encystment, but this 

 effect is produced by the stimulus to growth and division given 

 the organisms by changing the medium suddenly, and is not 

 due to the removal of an adverse environmental factor created 

 by the presence of metabolic by-products. This is quite com- 

 parable to seeding a new culture. 



The reader will doubtless have thought of the fact that an ever 

 plentiful supply of food was present during the early course of 

 Di, D2, D$, Ei, E2, and 3. However, the same arguments that 

 applied to metabolic by-products will apply here and we can 

 likewise conclude that food deficiency is not the cause of encyst- 

 ment. This was carried further by filtering thriving cultures 

 and replacing their medium with distilled water, 25 per cent, 

 hay infusion and 75 per cent, distilled water, etc. to pure hay 

 infusion, a change of 25 per cent, in each step. In trie cultures 

 that received pure distilled water no encystment took place and 

 the cultures died out in two days. In the others, however, 

 neither fission nor encystment seemed to be materially affected. 



A few experiments were carried out which may have involved 



