CONDITIONS FOR CONJUGATION 375 



uniform and favorable for growth, the process might probably 

 be postponed indefinitely (cf. Woodruff's culture). 



7. Many of these races which do not conjugate can be rendered 

 susceptible to conjugation by subjecting them to a prolonged 

 period of reduced nutrition, thus inhibiting multiplication. 

 Such 'dormant' cultures, of any race, when renewed by supplying 

 abundant nutriment, may yield conjugants more readily (if 

 conjugation occurs at all) than control cultures of the same strain 

 which have been kept under conditions of constant growth. 



8. The addition of certain salt solutions to samples taken 

 from these renewed cultures after a few days of growth may give 

 an increase in the relative number of conjugants in certain races, 

 without affecting appreciably the proportion of conjugants in 

 other races of the same species (as worked out especially for Par. 

 caudatum). 



9. A small amount of sodium nitrate added to the renewed 

 cultures at the time of their renewal, serves to augment conjuga- 

 tion in many races of Par. aurelia. Diverse races appear to 

 react differently toward equal concentrations of the salt as used 

 in this way, for race lb and its derivatives underwent conjuga- 

 tion more readily in aqueous medium than in the solutions of 

 the salt in concentrations found most effective for the other 

 races of this species. This same race multiplied more steadily 

 and maintained its numbers longer in aqueous cultures than other 

 races of this species. 



10. There is a progressive tendency, probably in all races, 

 for conjugation to become suppressed after prolonged cultiva- 

 tion under laboratory conditions (p. 368). The susceptibility 

 of such cultures in which conjugation is in abeyance can often 

 be partially restored by subjecting them to a long period of 

 dormancy, as described under point 7. 



-11. This restored power of conjugation in organisms subjected 

 to long dormancy is correlated with their higher rate of fission, 

 as compared with those organisms of the same race which have 

 previously been undergoing division. 



12. From what is known regarding the effects of salts in accel- 

 erating fission and in augmenting conjugation, it appears that 

 conjugation is initiated by a period of unregulated division. 



THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 34, NO. 3 



