REJUVENESCENCE IN UROLEPTUS MOBILIS 151 



the first period of sixty days after emerging from the cyst, it had 

 a higher division rate than any other representative of the A 

 protoplasm, the protoplasm of each line dividing, on the aver- 

 age, 19.8 times in ten days. If the vitality of the encysted in- 

 dividual was the same in potential as that of the same proto- 

 plasm in the isolation cultures, we would expect the division rate 

 of the isolation series during the sixty days subsequent to the 

 45th generation to be practically the same as that of the proto- 

 plasm from the cyst. This expectation, however, was not real- 

 ized, for during this period the F series divided only 17.6 times 

 in ten days — a difference in potential of 2.2 divisions in ten 

 days. The difference between the division rate of the F series 

 for its first sixty days and the M series for its first sixty days 

 was 2.6 divisions in ten days (table 4). 



The difference between the division rates of the parent F 

 series and the offspring M series by parthenogenesis is not actu- 

 ally as large as the figures indicate. The M series was main- 

 tained under the conditions of a constant temperature of 24°C., 

 while the F series was maintained under the conditions of lab- 

 oratory temperature which varied from 19° to 22°C. That the 

 difference in rate is not due solely to these different conditions, 

 however, is shown by a comparison of the M series with the L 

 series (cf. Table 2). The latter came from the I series and the I 

 series from the F series. Both L and M, therefore, had the 

 same ancestry. Furthermore, L and M were started at approxi- 

 mately the same time, L as an ex-conjugant, M from a cyst, and 

 both series were maintained under the same temperature condi- 

 tions. The L series, during the same sixty days as above for 

 the M series, had an average division rate of 18.8 in ten days 

 as against 19,8 for the M series. 



The unpedigreed B series, which came from a cyst, may be 

 compared with the C series which started as an ex-conjugant at 

 about the same time as the B series. The first sixty days of the 

 B series gave an average division rate of 17.4 divisions in ten 

 days, while that of the C series was 17.2. 



So far as the evidence thus far obtained is concerned, it ap- 

 pears that the initial vitality after encystment and partheno- 



THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 29, NO. 2 



