no 



S. O. MAST AND YASUSHI IBARA. 



mecia was now taken from each of the nine cultures and put into 

 nine square watch-glasses, each containing five vigorous didinia, 

 all taken from the same jar. These watch-glasses were put into a 

 thermostat kept at 27-28. The number of cultures in which 

 cysts occurred was recorded daily, as well as the number of cysts 

 in each and the condition of the paramecia. 



Other sets of watch-glass cultures were prepared and treated 

 precisely like this on the following days. A summary of the re- 

 sults obtained in all of these are presented in Table II. 



TABLE II. 



THE RELATION BETWEEN THE AGE OF THE FOOD-CULTURE AND ENCYSTMENT 



IN DIDINIUM. 



By referring to this table it will be seen that as the culture 

 medium increased in age the percentage of the number of cultures 

 in which encystment occurred increased to a maximum, after which 

 it decreased. It is well known that the chemical composition of 

 protozoa cultures changes with age. It is consequently evident 

 that the increase and decrease in the percentage of encystment noted 

 must have been due either to this change or to a change in the 

 quantity or the quality of the food. The amount of food was, 

 however, practically the same in all of the didinia cultures, but the 

 quality may have been different. It is therefore impossible to say 

 whether the change in the percentage of encystment was due to a 

 change in the chemical composition of the culture medium or to a 



