CONDITIOXS FOR COXjrGATIOX 349 



again on the twenty-second. Xot one was observed in oa. 

 Attempts were made on October 17th and October 22nd) to 

 induce the occmrence of conjugation in large numbers by sub- 

 jecting subcultures to the action of 0.00002 X FeCIs and of 

 0.001 X XaXOs solutions, but the results were negative. It 

 should be remembered, however, that these animals taken from 

 stock cultures had been growing iu a medium made up with 

 'tap-water.' and so perhaps might not have responded in the 

 same way as animals in a pure aqueous medium when treated 

 with salts. These same stock cultures were then allowed to 

 undergo another long period of donnancy > which may be dated 

 from Xovember 1, 1919). Two subcultures taken from these 

 stock cultures and renewed with spring water and hay infusion 

 on December 12th gave, in the case of oa. one pair of conjugants, 

 in ^a, a considerable epidemic of conjugation (1 to 5 per cent) 

 extending over a period of thirty-three days. Attempts to 

 augment the intensity of conjugation in. the latter culture by 

 treating samples with different concentrations of FeCL 0.00002 X, 

 0.00004 X. and O.OOOOS X)-. with KCl (0.00025 X and 0.0005 X) 

 and with XaXOs 0.001 X^ were ineffective: that is. the relative 

 number of paii^s obtained in these experiments was not greater 

 than in the renewed culture. 



A quite different state of afl"airs was found to ensue during 

 the later history of 17a. As pointed out above, this race proved 

 susceptible to conjugation during its early development. Cul- 

 tures 17al. II. and /// taken at different times from the same 

 stock culture which was started on January 17. 1919. from one 

 individual) yielded conjugants very readily when renewed after 

 two or three weeks of semistarvation (tables 1 and 2). The 

 general eff'ect of adding salts to the medium was to intensify con- 

 jugation in accordance with the results of Enriques and Zwei- 

 baum. After four months of cultivation, however, no more 

 conjugants were obtained in the experiments with this race. 

 The culture 17 aV. derived from 17 all I ^^hich had been con- 

 tinued as the stock culture\ when renewed on June 2Sth after 

 lying dormant for one month, gave no conjugants;, and experi- 

 ments with salt solutions (FeCls and XaXOs), such as were 



