358 HOYT S. HOPKINS 



except after fasting for eight to nine weeks. Others, on the 

 other hand (e. g., 13a, table 4, and 25a), conjugated after shorter 

 periods of dormancy — about three to four weeks. The duration 

 of this period serves perhaps as the best criterion -for judging 

 the relative susceptibility of various races toward conjugation, 

 for experiments with salt solutions generally give negative 

 results unless the organisms have become susceptible to conjuga- 

 tion previously through dormancy. 



The effects produced by using salt solutions directly in 

 experiments designed to induce conjugation (method of Zwei- 

 baum) are rather variable and unpredictable. As a rule, the 

 salt solutions increase the percentage of conjugating pairs, 

 but I suspect that their action is largely osmotic rather than 

 chemical. When added to the main cultures when renewed, 

 however, the effect of sodium salt is more marked, although 

 even by this method a relatively small number of conjugants 

 is obtained. As examples of this effect may be cited experi- 

 ments with race 13a, table 4; 14a, table 5; Hi and Uj, table 6, 

 and 3d, table 7. The reactions of these various races, however, 

 may be contrasted with that of race Ih, table 7, in which conjuga- 

 tion is not favored by the presence of sodium nitrate, or, if so, 

 only by a lower concentration than that which was found most 

 effective in the other races. 



In order to bring out more clearly some of the racial dif- 

 ferences between the various strains of P. aurelia, the following 

 observations are recorded. They indicate, also, something 

 regarding the nature of the 'periodicities' in conjugation so 

 characteristic of many strains of this species. 



Par. aurelia, 1 (6 and c) . This race was descended from a single 

 individual isolated January 30, 1919, from an old laboratory hay 

 culture. The original culture, lb, developed very rapidly under 

 laboratory conditions, the animals proving to be quite hardy at 

 all times during their history. 



A watch-glass culture containing numerous individuals was 

 set aside on March 17th, and on the following day showed con- 

 jugation involving about 10 per cent of all. Observations failed 

 to reveal the occurrence of the process in the stock culture 



