372 Problem of Rejuvenescence in Protozoa [June-September 



Enriques studied the same general problem and reached the con- 

 clusion that the degeneration and death of infusorian cultures was 

 due to bacterial poisons, because he succeeded in breeding Glaucoma 

 scintillans for 683 generations without signs of degeneration when 

 he took measures to ehminate this factor. Whether this was the 

 crucial factor in his method is open to question, but the significant 

 fact remains that his animals survived nearly twice as long as thöse 

 of earlier workers, without conjugation or artificial Stimulation, thus 

 suggesting that, if suitable conditions are supplied, reproduction by 

 division can proceed indefinitely. 



At this point I took up the problem and first investigated the 

 possibility that the degeneration observed in the previous investiga- 

 tions was induced by too great uniformity in the conditions of cul- 

 ture, or bythe culture-medium being deficient in something essential 

 for the continued well-being of the organisms. A race of Para- 

 mcecmm aurelia was isolated in 1907 and bred on infusions of vari- 

 ous materials found in the natural environment of the animal, while 

 a sub-culture was subjected to the relatively constant hay-infusion 

 culture-conditions generally employed. The result was that the cells 

 bred in the constant hay-infusion medium died out after a typical 

 Calkins cycle, while those bred on the ' varied-environment ' medium 

 did not pass through periods of marked physiological depression or 

 show morphological changes which could be interpreted as abnormal. 

 This race is still, after more than eight years in culture, in a normal 

 condition, having attained over 5250 generations without conjuga- 

 tion or the use of artificial Stimuli. 



The success with the varied culture medium naturally led to the 

 question whether the longevity of Paramcscium, on a varied environ- 

 ment, is dependent upon intrinsic Stimuli from the frequent changes 

 of the medium, or whether a constant medium of hay-infusion is 

 unfavorable because it lacks some Clements which are essential for 

 the continued existence of the organism. Accordingly,^ a sub-cul- 

 ture of this race was bred for a period of nine months on a constant 

 culture-medium of beef-extract. The continued health of the or- 

 ganisms on this constant medium throughout the experiment, which 

 was continued sufficiently long to include a Calkins cycle, if such 



1 Woodruff and Baitsell: Journ. of Exp. ZooL, 1911. 



