CONDITIONS FOR CONJUGATION 353 



its appearance on February 28th in one, on March 2nd in the 

 other. It made its first appearance in several exconjugant 

 strains of the fourth 'generation' (^^c?, isolated on February 

 28th from a culture of 44c) as early as the 23rd of March. In 

 the original culture, Ha, after the occurrence of conjugation 

 during November and December, no more conjugants were 

 observed until March 12, 1920, when they were found to the 

 extent of about 5 per cent. Casual pairing occurred subse- 

 quently during a period of several days. 



It is important to compare the reactions of these animals of 

 race 44 with those of race 38 during their respective periods 

 of conjugation. In the latter strain a shght disturbance in 

 culture conditions (addition of hay infusion) caused immediate 

 cessation of the process, whereas in the former it exercised but 

 a temporary and partial influence when several times repeated. 

 We have, as it would appear, two strains in which conjugation 

 tends to manifest itself periodically, in one of which it is largely 

 influenced through the enviromnent, in the other more nearly 

 independent. 



The two races J^Sa and 57a were derived as exconjugants froni 

 the same source as If-Ji-a, on the same day (September 25, 1919). 

 They were cultivated under parallel conditions and examined 

 regularly during a period of six months, but no conjugants were 

 observed in either of these continuous cultures grown under 

 the same conditions as Jf-J^a. During the course of the experi- 

 ments described above conjugation occurred in the renewed 

 culture of 57a, but not in as great numbers as in Ij-Ji-a, whereas 

 no conjugation took place in 45a after renewal. 



In the remaining race, 8a, used in these experiments, conju- 

 gation has never been observed, either in the experiments 

 or in a continuous culture maintained under the same uniform 

 conditions as the continuous cultures of the six other strains 

 (listed in table 3). 



It is apparent from the results of these observations and 

 experiments with several strains of Paramecium caudatum that 

 no general directions may be formulated by which we can bring 

 about conjugation in a given strain at any one time. When 



