STUDIES ON LIFE-HISTORY OF PROTOZOA. 1 93 



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argues that, like Metazoa, the race would show unmistakable signs 

 of senile degeneration and would die of old age were it not for 

 conjugation which, in some unknown way, restores the vitality 

 and ensures a new cycle of generations. According to Maupas, 

 therefore, it would seem that the protoplasm of the unicellular 

 organisms is not capable of the perpetual exercise of the ordi- 

 nary vital functions. His conclusions lead to the assumption 

 that, after conjugation, the individuals have a certain potential of 

 vitality which is gradually used up in the course of a more or 

 less definite number of generations, and which can be restored 

 again and again by conjugation. 



The general theories involved in this discussion seem to me im- 

 portant enough to warrant further experimental tests, and with 

 this in mind, I started the culture of two different lines of Para- 

 mccciujn caiidatinii, on the ist of February, 1901, and these have 

 been under continual observation ever since. One of the lines 

 died out last month, in the 57Oth generation, the other is still 

 living in the 665th generation. 



One definite aim at the outset was to ascertain whether a given 

 form under culture would pass through a more or less clearly- 

 marked cycle of activity to die ultimately of old age as Maupas 

 maintained. Another aim was to find out whether there is a 

 well-marked degeneration of different parts of the cell as age 

 advances. These aims merely called for the repetition of 

 Maupas' work in the case of one organism and were interesting 

 enough in themselves. There was still another aim, however, 

 which lent added zest to the work and made every stage one of 

 fascination. This aim was to find out, if possible, the significance 

 of conjugation and why it is that vitality is renewed by this 

 means. It seemed to me that if Professor Loeb could induce 

 artificial fertilization in the echinoderm egg, it should be possible 

 to induce artificial rejuvenescence in Paramaecium. The first 

 definite problem that suggested itself was the discovery of some 

 substitute for conjugation that could be used when the periods 

 of physiological depression were imminent. The experiments 

 have justified the assumption that such substitutes would prove 

 successful, for I have been able to carry these minute organisms, 

 not through merely one period of so-called degeneration, but 



