340 BIOLOGY OF THE PROTOZOA 



striking illustration is shown by Series 7 and its two offspring, Series 

 9 and 14; Series 9 came from Series 7 when the latter had lived more 

 than half of its life and its relative vitality was about 15 per cent 

 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. 



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 Spat Iridium spathula. 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 (56 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 aurslia 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. Different 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 (Woodruff, 1917). According to Jollos 

 (1916) external factors may call out parthenogenesis in Paramecium 

 at any stage in the life history, and according to Young (1917) sud- 

 den 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 c/ua 

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



With Uroleptus mobilis 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 



