580 BIOLOGY OF THE PROTOZOA 



Also there has })een a slow but decided change in the extent of 

 rejuvenescence after conjugation. The average division-rate per 

 individual per series for the first sixty days of life of the first 50 

 series was 15.7 divisions in ten days; for the last 50 series it has 

 fallen to an average of 13.1 divisions per ten days. While this 

 would apparently indicate a weakening of the organization of 

 Uroleptus, such is not the case, for along with this change in intensity 

 has gone a change in the length of life in division days. The 

 average length of life of the first 60 series was two hundred and 

 ten days of the last 60 series two hundred anfl sixty-three days. 

 The relative vitality has not changed although there has been a 

 slight change in the manifestations of that \itality. The initial 

 sixty days is no longer always the period of optimum vitality as was 

 the case originally, but the period of greatest vigor now occurs 

 after from one hundred to one hundred and twenty days and this 

 vigor is retained for a longer stretch of the life cycle (Fig. 232). 

 Such changes are gradual and imperceptible and are demonstrable 

 only by an analysis of a mass of accumulated data. 



While the above conclusion is true for Uroleptus generally, it 

 does not preclude the possibility of sudden sports or mutations. 

 These might well be expected in a succession of more than 100 

 fertilizations. While no morphological changes have occurred in 

 this manner some evidence is afforded by abnormally vigorous 

 series and by exceptionally weak series. The former are represented 

 by Series 89, 19 and 45 (see Table, p. 560). The first of these 

 (Series 89) with a relative vitality of 103.6 per cent was the double 

 organism already described (p. 465). This can scarcely be called 

 a mutation although it was a real case of merogamy with double 

 amphinuclei, and the zygote continued to breed true by uniparental 

 inheritance for 367 generations. It ne\'er formed cysts and never 

 conjugated but died after four himdred and fi\'e days of life. vSeries 

 19 with a relative vitality of 110.4 per cent was quite unusual. It 

 lived for five hundred and ninety-eight-division days and divided 

 597 times and its curve of vitality was of an entirely different 

 type from other series of Uroleptus (Fig. 237). It arose as a third 

 generation of a set of old-age conjugations and a relatively low 

 vitality was to be exj^ected. The opposite result must have been 

 due to some combination possibly associated with conjugations of 

 old parents and grandparents. Its peculiarities were not handed on 

 to its progeny (Series 41, 43 and 44). It is possible that Mast's 

 (1917) "mutation" in Didiniuvi nasiihivi and Woodruff's long-lived 

 series of Spathidium spatlmla were analogous cases of unusual 

 germinal combination (Woodruft' and Moore, 1924). We do not 

 agree with Woodruff in interpreting our Series 19 as he does his 

 unusual race of SjKifhidiuvi as due to a fortiniate aggregate of 



