564 BIOLOGY OF THE PROTOZOA 



lower than its parent. Series 14 came from the same parent when 

 the latter had only 6 more divisions in its life history and the effect 

 of its old age is shown by the relative vitality of 5.4 per cent of 

 its offspring, Series 14. It is quite evident that the protoplasmic 

 organization of the parent is not the same at the beginning and at 

 the end of its life and that the effect of the change is indicated by 

 the organization and activities of its offspring. Some interesting 

 and perhaps significant surprises have turned up however from 

 such old age conjugations and it is possible that mutations may 

 arise at such times. Thus Series 19 came from parents that were 

 225 generations old and with only 32 more generations to live. The 

 expectation would be a low relative vitality for this old age offspring, 

 but on the contrary it had a relative vitality of 110.4 per cent, the 

 highest on record (see p. 580 for further consideration of this case.) 



In our experience it has been impossible to restore an extremely 

 weak series to a vigorous condition by conjugation, all such attempts 

 result in still weaker series. It is possible, however, to restore com- 

 paratively weak series to full strength, a result which Woodruff and 

 Spencer also obtained with Spathidium spathida. This is well 

 shown by Series 60 and 62, in which the relative vitality is raised 

 from 70.3 to 96.4, or by Series 66 and 70, in which it is raised from 

 69.1 to 95.0, etc. 



4. Rejuvenescence After Parthenogenesis (Endomixis).— Woodruff's 

 long culture of Paramecium aurelia furnishes an excellent illustration 

 of continued vitality through reorganization by parthenogenesis. 

 The fluctuations or waves in his graph (Woodruff 1921) indicate 

 a series of depressions followed by increased vitality; reorganization 

 occurs during the periods of depression. Diflerent culture media 

 have no effect in changing the frequency of endomixis in time but 

 may cause an increase or decrease in the number of interendomictic 

 generations by divisions (Woodruft", 1917). According to Jollos (1916) 

 external factors may call out parthenogenesis in Paramecinm at 

 any stage in the life history, and according to Young (1917) sudden 

 sharp changes of medium may bring on endomixis prematurely, 

 but the sequence always lapses to the regular routine and usually 

 by the next period. If endomixis does not occur the race invariably 

 dies. "This indicates strongly, if it does not prove that a periodic 

 occurrence of the definitive endomictic phenomena is a sine qua 

 nun for the continued life of the race" (Woodruff, 1917, p. 462). 



With Uroleptvs mohilis the evidence for rejuvenescence through 

 parthenogenesis is of the same kind as that from conjugations. 

 Reorganization without fertilization takes place during encystment 

 and the cysts are formed early in the life history of a series (see 

 p. 490). On emerging from its cyst the organism is treated as 

 though it were an ex-conjugant and the first fi^T indi\'iduals are 

 maintained as fi\e pure lines of the series. Such series are indicated 



