UROLEPTUS MOBILIS ENGELM. 469 



maturation stages leading to reorganization and rejuvenescence 

 and that any stimulus will start the machinery. Experiments 

 in cutting off the anterior ends of individuals ready to conjugate 

 result in no reorganization processes as stated above. The 

 shock of cutting, therefore, had no effect in starting the series 

 of nuclear and cytoplasmic changes characteristic of conjuga- 

 tion. We thus limit the possible time of stimulation to some 

 period between the sexually mature prefusion individual and the 

 beginning changes of the micronuclei after fusion of the two 

 individuals; that is, shortly after contact of the two conjugating 

 cells. 



In picking out a pair of conjugating individuals with a pipette 

 it sometimes happens that the individuals have only recently 

 come together and are not firmly fused. Such organisms enter 

 the pipette as a pair, but upon transferring to another culture 

 dish they emerge from the pipette as two single individuals. 

 This is an extremely rare occurrence, however, and I have vainly 

 tried by repeatedly spurting them from the capillary pipette 

 to separate individuals which I had reason to believe had 

 recently come together. Such splitting of pairs is comparatively 

 simple with Paramecium caudatum, but with Uroleptus I 

 have never succeeded in doing it at will. On one occasion, 

 however, when a pair had separated in the pipette without con- 

 scious effort on my part, the two individuals were isolated in 

 separate culture dishes and treated as were the cut conjugants. 

 This occurred with a conjugating pair during a conjugation- 

 epidemic in the L series which was in the 203rd generation on 

 March 15,1919. One individual lived for thirteen, the other 

 for fourteen days before they died, but during these two weeks 

 each cell passed through the different phases of nuclear reorgani- 

 zation exactly as though conjugation had taken place. These 

 phases, reproduced from sketches made while the cells were 

 alive, are illustrated 'in figure 10. 



Although these individuals died, we have here unmistakable 

 evidence that the entire series of maturation and reorganization 

 processes were set in motion by the first contact of the two 

 individuals in conjugation. They were firmly enough united to 



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



