CONJUGATION AND ENCYSTMENT 351 



many generations thus it was almost impossible to induce en- 

 cystment. Calkins obtained even a more marked effect of 

 continued culture on encystment. He says ('16, p. 266): ''In 

 my experiments with Didinium the race apparently lost its 

 power to encyst and ultimately died out after six months' cul- 

 ture without encystment." If there really are in Didinium, 

 cycles ending in encystment as Calkins maintains, they are very 

 long and if encystment results in better adjustment to the 

 environment during periods of depression, if it is a rejuvenating 

 process, why was there not a strong tendency to encyst near 

 the close of our experiment when the \dtality of the didinia was 

 low, instead of no such tendency whatever? 



Calkins also obtained other results which do not seem to be in 

 harmony with his conclusions. He says ('15, p. 238): 



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 recoveiy 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. 



In the first cycle referred to the didinia produced 131 genera- 

 tions and then encysted; in the second they produced 148 genera- 

 tions. In each case conjugation occurred shortly after the 

 didinia came out of the cysts. In my work I observed repeatedly 

 a strong tendency to conjugate shortly after encystment, thus 

 confirming the results obtained by Calkins. But if encystment 

 and conjugation have the same function, if both are rejuvenat- 

 ing processes, why was there such a strong tendency to conju- 

 gate soon after ''recovery from encystment?" And why was 



