340 HOYT S. HOPKINS 



That the process may be postponed indefinitely in Paramecium 

 aurelia, provided the cultural conditions are kept uniform and 

 favorable, seems probable in the light of Woodruff's extended 

 work, for he maintained one strain through more than six thou- 

 sand generations without conjugation (1917). This same race 

 of organisms was subjected to conditions which favor conjuga- 

 tion with positive results (Woodruff, '14). It would appear 

 from these results that, in Par. aurelia at least, the occurrence 

 of conjugation is facultative under uniform conditions of environ- 

 ment. Other reports, moreover, would indicate that conjuga- 

 tion is possibly lacking in the life-history of some races of Para- 

 mecium, or recurrent only after very long time intervals in 

 others. Thus Jennings ('10) maintains that there exist in these 

 organisms striking racial differences with respect to conjugation. 

 In some of the strains employed in his work normal 'epidemics' 

 occurred frequently, sometimes apparently under uniform condi- 

 tions of existence, in some only infrequently, and in others not at all. 

 He emphasized also growth and nutrition as important factors 

 in regulating conjugation. Thus a period of rapid multiplication 

 followed by a decline in the division rate usually gave an epidemic 

 of conjugation in the majority of cases. A preliminary period 

 of semistarvation (reduction in division rate) of two weeks or 

 longer made the organisms more susceptible when the above- 

 mentioned procedure was followed. Some few cultures (races) 

 failed to yield conjugants even when the most favorable condi- 

 tions were afforded. Two species of Paramecium (caudatum 

 and aurelia) were used in his work, and the same kind of differ- 

 ences was found in each: there were races which conjugated 

 freely, often at frequent intervals, others which did not, although 

 P. caudatum showed less frequent epidemics and was character- 

 ized by a greater number of the 'non-conjugating' races. 



The process of nuclear reorganization, or endomixis, discovered 

 by Hertwig ('89), and worked out by Woodruff and Erdmann 

 ('14, '16), would seem to suffice in maintaining the normal 

 vegetative processes in Paramecium in the entire absence of 

 conjugation. Even in such a form as the hypotrichate Uro- 

 peltus, according to the work of Calkins ('19, two papers). 



