356 HOYT S. HOPKINS 



maintain their ability to conjugate freely for several 'generations/ 

 in new cultures started from exconjugants of a previous epidemic. 



b. Paramecium aurelia 



My studies upon races of this species open up problems essen- 

 tially like those presented by caudatum. Some of the cultures 

 were subjected to daily observation during the first weeks of 

 their development. Conjugation was thus shown to be of fre- 

 quent periodic occurrence in some, rare and apparently sporadic 

 in others, while in some it was found neither during this period 

 of daily observation nor on subsequent examination at intervals 

 of two to five days for several months. At intervals dense 

 watch-glass cultures, derived from the main cultures, were set 

 aside in moist chambers and examined the following day for 

 conjugating pairs. In such cultures the nutriment is quickly 

 used up by the paramecia, and the conditions are right for an 

 epidemic of conjugation, provided the organisms were taken 

 from a flourishing culture (method of Maupas, '89). In this 

 manner some of the races which showed no conjugants in the 

 stock cultures were made to give several in these isolation experi- 

 ments, while others, which showed small numbers in the stock 

 cultures, yielded as many as 50 per cent in the experiments. 

 As a rule, pairing took place earlier and persisted later during 

 a series of such experiments than in the main cultures themselves, 

 when the experiments were begun sufficiently in advance of an 

 epidemic of conjugation in the main culture. That is, conjuga- 

 tion could generally be obtained in the dense watch-glass 

 cultures several days before any pairs were seen in the stock 

 culture from which they had been derived, and could be obtained 

 for days after it had ceased in the stock culture. The effect 

 in such experiments is, then to augment conjugation in cultures 

 of organisms already in a state of susceptibility. Certain 

 races, however, failed to conjugate during a period of several 

 months, even when placed under the more favorable conditions 

 mentioned above (isolation experiments). In this connection, 

 also, solutions of various salts were added to some of the watch- 

 glass cultures when isolated. This served to increase somewhat 



