Rejuvenescence in Uroleptus. 



17 



proximately 100 generations) the average difference in vitality 

 amounts to only 1.5 divisions in 10 days for the first 60-day period 

 of the offspring. If the filial generation starts from a parent 

 series which is 130 to 200 generations old, the difference in vitality 

 between parent and offspring increases to between 3 and 4 divisions 

 in 10 days, and if the filial generation starts from a parent series 

 that is about 300 generations old, the difference in vitality rises 

 to a maximum of 17 or more divisions in 10 days. 



In the present experiments the parent P series was in the 

 140th generation when the filial V series was started, and in the 

 160th generation when X6 and X7 were cut. The difference in 

 vitality between the V series and the P series, 3.1 divisions in 

 10 days for the first 60 days, agrees with the results obtained with 

 all ex-conjugants started at corresponding periods of the parent 

 cycle, and shows the normal rejuvenating effects of conjugation. 



The cut conjugants show different results in regard to renewal 

 of vitality. The X series shows no rejuvenescence at all but 

 agrees with the non-conjugant P series throughout. The X6 

 series agrees with the non-conjugant P series in the first 60-day 

 period but has a higher average division rate for the second 60- 

 day period, while its average for four months is intermediate be- 

 tween the non-conjugant P series and the ex-conjugant V series. 

 The X7 series, on the other hand, shows for the entire period the 

 same vitality as the ex-conjugant V series and gives the same 

 evidence of rejuvenescence. 



These three divergent cases are difficult to interpret. Each 

 recovered perfectly after the operation; each underwent reor- 

 ganization processes apparently identical with those of normal 

 ex-conjugants; each gave rise to a population similar to that of 

 a normal ex-conjugant except that in none of them have I seen 

 a case of encystment; epidemics of conjugation have been fre- 

 quent in each of these populations. 



A possible explanation may be found in the fact that the con- 

 jugating pairs which were operated upon to give the X, X6, and 

 X7 series were cut at different phases of the process of conjugation. 

 In none of them had nuclear interchange taken place. One pair 

 was cut at the very outset of conjugation; this gave rise to the 

 X series which showed no rejuvenescence. Another pair was cut 



