ENCYSTMENT OF DIDINIUM NASUTUM. 



In all of these experiments the didinia were rather suddenly sub- 

 jected to the different temperatures. It was thought that the lack 

 of encystment at the extreme temperatures might have been due to 

 the sudden change. An extended series of tests was consequently 

 made in which the temperature of some of the cultures was very 

 gradually reduced to 4-i6, and that of others very gradually 

 raised to 39. The results obtained in all of these tests were, how- 

 ever, essentially the same as those obtained in the earlier experi- 

 ments. 



All of the results obtained consequently indicate that encystment 

 in Didinimn takes place most readily under conditions of tempera- 

 ture and food which appear to be optimum for reproduction. The 

 results are, however, not conclusive in reference to the question of 

 the effect of food. An abundance of food was added in all of the 

 experiments when they were set up and in some more was added 

 later. In some cultures there was still an abundance of food pres- 

 ent when encystment occurred, but in others there was none. The 

 number of each was unfortunately not recorded. It is conse- 

 quently evident that in some of the cultures which contained food 

 encystment may have been due to absence of food. In the cultures 

 with food there was a much greater increase in the number of 

 didinia than in those without food. Rapid and extensive repro- 

 duction of didinia confined to a small space seems to favor encyst- 

 ment, and it may be that this is owing to accumulation of waste 

 products. 



In attempting to ascertain the effect of the age of the food-cul- 

 ture on encystment experiments were carried out as follows : Tim- 

 othy hay was added to tap-water, spring-water, and distilled water 

 in three large flasks, one gram to 100 c.c. These flasks were then 

 kept for 30 minutes at the boiling point. After they had cooled to 

 room temperature, usually the following day, a large number of 

 paramecia in a small amount of liquid was added to each flask and 

 the contents poured into battery-jars. These jars were then placed 

 side by side and kept at room temperature. Three more cultures 

 were prepared precisely the same way two days later and also four 

 days later ; so that at this time there were at hand three sets of 

 paramecia cultures, one just completed, one two days old, and one 

 four days old. A given amount of solution containing many para- 



