EXCYSTATION 57 



such a view it would be necessary to assume that 

 a cyst would develop when placed in pure water 

 only, in which case the escaping organism would 

 either die of starvation, or, if this theory were 

 correct, would encyst again. We now know by 

 actual experiment that neither of these events 

 takes place (see later). Excy station does not take 

 place without the influence of certain bacterial 

 products (the nature of which we have to some 

 extent been able to ascertain) acting on the cyst 

 from without, nor does encystment occur in the 

 absence of other products, which appear to be 

 of a poisonous nature. Secondly, the excysta- 

 tion may be attributed to the fact that the term 

 " suitable conditions " really means the addition 

 of a fresh supply of food or something which was 

 absent in the old medium. In other words, it is 

 now the presence of some added stimulus in con- 

 trast to the absence of harmful substances which 

 was expressed by the first view. We find from 

 our experiments, however, that there is no question 

 some added stimulus is essential for excystation to 

 take place ; and although we have not adduced any 

 positive evidence that removal of injurious pro- 

 ducts is necessary, it can safely be assumed that 

 excystation could not occur in the presence of a 

 sufficient degree of concentration of those sub- 

 stances which cause the encystment. As a matter 

 of fact, we find that it is the presence of food, 

 namely bacteria, which acts as the stimulus to 

 excystation ; but its action is not due to its mere 

 existence in the neighbourhood of the cysts, but 

 is felt through the production of chemical sub- 

 stances which are capable of weakening the cyst 



