CONJUGATION AND ENCYSTMENT 343 



frequently does not occur in the absence of food, no matter how 

 many are present or what temperature is employed. In fact I 

 was quite unable in many instances to induce conjugation. It is 

 evident, therefore, that conjugation depends upon internal as 

 well as upon external factors, but what these factors are is as 

 yet unknown. The results of my experiments seem to indicate 

 that conjugations can be induced more readily in lines in which 

 it has been prevented for long periods than in lines in which 

 it has recently occurred. They indicate, however, that there 

 is no specific relation between the tendency to conjugate and the 

 number of consecutive asexual generations produced in a line; 

 for in several instances I obtained conjugation in abundance in 

 lines which had produced less than 100 consecutive asexual 

 generations and in others I was unable to induce conjugation in 

 lines which had produced more than 1000 such generations. 

 Moreover, in certain lines in which conjugation occurred among 

 the accessory specimens at a given time it was not possible to 

 induce conjugation during certain later periods, i.e., after more 

 asexual generations had been produced. Furthermore, Calkins 

 ('15, p. 238) obtained conjugation in individuals which had pro- 

 duced only two asexual generations; and Jennings ('10) obtained 

 similar results in Paramecium. 



THE EFFECT OF CONJUGATION AND ENCYSTMENT ON THE RATE 



• OF FISSION 



All of the evidence obtained on the relation between fission- 

 rate and conjugation and encystment is presented in table 1. 

 This table contains a brief history of all of the groups of pure 

 lines that were studied. In it are given the time each group 

 was isolated in relation to the time of conjugation and encyst- 

 ment, the ancestry and duration of each, the number of lines 

 each contained and the average fission and death-rates. 



An examination of table 1, with some explanatory additions, 

 shows the following: In April, 1910, two groups of four lines each 

 were isolated, one immediately, the other some time after con- 

 jugation. After these two groups had continued for thirty 

 days the latter was closed. During this time the fission-rate 



