REJUVENESCENCE IN UROLEPTUS MOBILIS 153 



J series, was able to divide at the rate of 17.9 times in ten days, 

 or 71.6 times in forty days, and there is no reason to doubt 

 that the other ex-con jugant of this pair would have had the 

 same vigor. 



The experiments also indicate that there is a limit to the ex- 

 tent to which this protoplasm can be rejuvenated. It might be 

 inferred, with reason, that if two weak individuals are trans- 

 formed by conjugation into individuals capable of dividing sev- 

 enteen times in ten days, then conjugation between two indi- 

 viduals from a series in which physiological weakness is not yet 

 perceptible, would result in an ex-con jugant capable of dividing 

 more than seventeen times in ten days. The inference, however, 

 is not supported by the facts. A good illustration is the rela- 

 tion between the F and C series (cf. Table 5). The F series came 

 from C when the latter's vitality was indicated by 17.2 divisions 

 in ten days. Each of the two individuals conjugating at this time, 

 would have had the ability to divide at the rate of 16.2 times in 

 ten days during the ensuing sixty days, if they had not conju- 

 gated. Having conjugated, they were able to divide at the 

 rate of 17.2 times — a difference, or extent of rejuvenescence, 

 indicated by only one division in ten days. Since all ex-con- 

 jugants, under the conditions of the experiments and regardless 

 of the state of vitality of the parent protoplasm, return to this 

 same optimum of vitality measured by 17+ divisions in ten 

 days (table 4), it is evident that the protoplasm of Uroletpus 

 will hold only a certain charge, so to speak, or potential of 

 metabolic vigor, as a result of conjugation. This optimum, of 

 course, is subject to change by changes in the environmental 

 conditions — heat, for example, increasing it. 



A third physiological fact is also indicated by these experi- 

 ments, although the numerical support is not as adequate as that 

 in support of rejuvenescence through conjugation. This is the 

 fact of rejuvenescence following encystment and parthenogenesis, 

 in which no nuclear interchange occurs. The B series and the 

 M series came from cysts. The ancestry of the former is un- 

 known, but during its first sixty days in culture, its metabolic 

 vigor was measured by a division rate of 17.4 times in ten days. 



