Studies on the Life History of Protozoa. 453 



questions may be asked was it old age from which the organism 

 died ? and, if so, what form did it take ? They were fed daily with 

 the same food upon which the stimulated sister cells thrived, but 

 they could not assimilate it and would not grow nor divide. In 

 similar cultures which had been carried to a like point by previous 

 observers, the entire race died, and although no evidence of 

 structural degeneration was evident, it has been taken for granted 

 that their organisms died from exhausted vitality, or in other 

 words, of old age. In my cultures there was some evidence of 

 degeneration, especially in the endoplasmic structures and in the 

 macronucleus. 



The fact that stimulation was successful in carrying the race 

 through this earlier period of depression indicates either that the 

 conditions are not the same as those accompanying old age in 

 metazoa, or else that such conditions may be satisfactorily over- 

 come. I believe the conditions are more or less the same in both 

 cases, and that in senile Paramcecta certain functions have become 

 retarded, possibly by the accumulation of useless protoplasmic 

 elements too minute to be detected, or by some less mechanical 

 cause connected with the molecular structure of protoplasm and 

 which, therefore, affords no morphological evidence of change. 

 Such an hypothesis would explain the difference in length of time 

 required to get positive results in the stimulation experiments. 

 For example, in August, 1901, after the race had been on hay 

 infusion continuously for 7 months, it was necessary to keep the 

 single individuals on beef extract for three weeks before they 

 would live again in the hay infusion. But in December it was 

 necessary to keep them on the stimulant only a day or two to get 

 the desired result. The short treatment at this period sufficed, 

 because they were not allowed to become weakened to the same 

 extent as in the preceding period of depression. This result 

 points to some physical condition of the protoplasm, possibly to 

 the accumulation of some protoplasmic product or products which 

 lead to diminished vigor and to death. Reinvigoration after a 

 more or less prolonged treatment with the beef extract and stimu- 

 lation by this and other means indicates that such materials are 

 disposed of, or, more generally speaking, and to use a phrase which 



