HIE CAUSE AND NATURE OF ENCYMMI AT. 41 



eft'ect on death rate, fission rate, or variation in fission rate. 

 This would indicate that neither of these processes is a re- 

 juvenating process, at least not in the sense in which Calkins 

 has used the term." 



The foregoing review certainly indicates that more experi- 

 mental work is needed on this subject, not only to obtain a better 

 understanding of the protozoan life cycle, but also for the 

 elucidation of the general biology of the so-called "resting forms 

 of protoplasm." With this in view we have undertaken the 

 work recorded in the following pages. The colorless phyto- 

 monad, Polytomella citri was selected as the organism to be 

 experimented upon for two reasons: (a) it encysts very readily, 

 and (b) most of the previous work has been confined to ciliates 

 and amcebse and we thought it desirable to extend the experi- 

 mental field to flagellates. 



We wish to express our gratitude to Professor E. G. Conklin- 

 not only for his criticism, but also for his interest and encourage- 

 ment. Our thanks are also due to Professor E. Newton Harvey 

 for some very valuable suggestions. 



MATERIAL AND METHODS. 



The culture medium used in all of the experiments described in 

 the following pages was made by boiling timothy hay in distilled 

 water, about ten grams of hay to a liter of water. Three liters 

 was the quantity usually made at one time, and all of the cultures 

 of any one experiment contained medium of a single making, 

 thus eliminating the possible error due to variation in the medium. 

 Test tubes served the purpose of containers. Approximately 

 15 cc. was the quantity of medium in each culture. 



Attempts were made to determine the effect of (i) various 

 temperatures, (2) hydrogen-ion concentrations, (3) metabolic by- 

 products, and (4) food deficiency on the tendency to encyst. 



(i) The different temperatures were chosen because of their 

 availability; and in spite of the fact that there was considerable 

 variation in some cases the results were sufficiently striking for 

 the present purpose. No attempt was made to arrive at an 

 optimum temperature within a narrow range, as this was not 

 thought to be necessary. 



