238 GARY N. CALKINS 



by substituting fresh water for the medium in which it had 

 encysted. Second, for reproduction, a phenomenon observed 

 in TilHna, Colpoda and a number of other cihates, but by no 

 means universal in the group. Third, for reorganization, 

 which has to do with internal processes of the cell. In Didin- 

 ium there is no encystment for purposes of reproduction, 

 but it is frequent for purposes of protection and periodic for 

 purposes of reorganization. In the latter case the approach 

 of encystment can be predicted very often from the reduced 

 activity in feeding and in dividing, from two to four days in 

 advance. This is shown not only by the averages for the entire 

 race but also by individuals and their progeny watched from 

 day to day. When in this condition, fresh water and food have 

 no effect, nor will fresh water added daily bring such individuals 

 out of their cysts until a period of at least five days has elapsed. 

 A very unexpected result was obtained in these experiments 

 in connection with the phenomena of conjugation. During the 

 first cycle no conjugating pairs were observed in any of the stock 

 dishes although such material is prepared daily and always 

 watched for at least five days. During the first week of the 

 second cycle, epidemics of conjugation appeared in the stock 

 dishes. This period of conjugation lasted about ten days, after 

 which not a pair was seen. Conjugation epidemics appeared 

 again in the third cycle and at a corresponding time. The first 

 pairs were seen in the stock dishes on the third day after recovery 

 from encystment (March 12) and pairings occurred in great 

 numbers until March 20th, after which not one pair could be 

 obtained from the material. During the height of the epidemic 

 in the stock material two cases of conjugation occurred in the 

 isolation cultures. One of these pairs (March 16) was the 

 union of two individuals out of eight derived from one individual 

 isolated the day before. The second case occurred on March 

 17 between two individuals among sixteen derived from an 

 individual isolated the day before. 



■ April 26, 1915 



