CONJUGATION AND ENCYSTMENT 345 



The didinia soon became numerous and in a few days many 

 conjugating pairs were discovered. In the meantime, they had 

 been examined at intervals short enough to make it certain that 

 conjugation had not previously occurred. Similar precautions 

 were taken throughout the entire series of experiments. 



From the culture containing the conjugating specimens five 

 groups of lines were started May 2, 1911, as indicated in the 

 table. The lines in two of these groups contained individuals 

 which had conjugated normally (ex-con jugants) ; those in one 

 group contained individuals which had been mechanically sepa- 

 rated immediately after they had joined in the process of con- 

 jugation (conjugants separated or 'split pairs'); those in another 

 group contained individuals which showed by. their size and 

 action that they were ready to conjugate (small) ; and those in 

 the fifth group contained individuals which showed no indication 

 of preparation for conjugation (large). 



The table shows that these five groups in the beginning con- 

 sisted respectively of 70, 20, 20, 13 and 10 lines, and that during 

 the first ten-day period the fission-rate was somewhat lower in 

 the two conjugating groups than it was in the three non-conju- 

 gating groups but that for the following three periods it was 

 remarkably nearly the same in all. 



At the close of these periods all of the active didinia died 

 owing to excessively hot weather during which the temperature 

 reached 35° several times. Cysts, however, which had formed 

 in some of the dishes retained for special study after isolating 

 individuals for propagating the lines, were not injured, and from 

 one of these, four new groups of lines were started. 



Thus, the experiment was continued, groups of new lines being 

 started from time to time, some from individuals which had re- 

 cently conjugated, some from conjugants separated, some from 

 individuals ready to conjugate, some from large specimens which 

 were not ready to conjugate and some from specimens which had 

 been encysted for various periods of time, as indicated in the 

 table. Consequently there were present throughout the entire 

 experiment continuously, several groups of lines which differed 

 markedly with reference to the number of generations since 



