Studies on the Life History of Protozoa. 455 



the cell appear to control, seemed to be equally normal, since the 

 organisms were of full size, while the endoplasm was full of 

 partly digested food. It appears, then, that the experiments 

 were successful in reinvigorating the elements of the cell that had 

 given out in previous periods of depression, but that other ele- 

 ments were now involved which all my experiments failed to 

 reach. Here a more deeply-lying malady had to be met, and the 

 experiments being unsuccessful in meeting it, the entire race 

 died out. This series of facts appears to warrant the assump- 

 tion that there is a fundamental difference in the protoplasmic 

 elements which go to make up the body of a protozoan, one of which 

 IS to be compared with the somatic cells of metazoa, the other tvith 

 germ cells; the one connected with the vegetative functions of 

 metabolism, the other with reproduction; the one may give out and 

 so lead to "physiological death" (Hertwig) or it may be restimu- 

 lated; the other may give out and so lead to "germinal death" of 

 the race. 



It is not outside the range of possibility that the last depression 

 period might have been overcome by some suitable experiments, 

 and the fact that we did not succeed in finding a suitable stimulant 

 does not justify us in assuming that this period represents the last 

 vital spark of this protoplasm, any more than we are justified in 

 assuming that the earlier periods of depression represented this 

 condition. If, however, some element or elements of the proto- 

 plasm become exhausted and all experiments to replace them fail, 

 then we might justly speak of exhaustion or "old age" of these 

 elements of the protoplasm and affirm that old age in one form, 

 characterized the organisms during the first two periods of de- 

 pression, while it took another form in the final period. 



3. Conjugation and Rejuvenescence. 



"Old age," then, appears to be a natural condition of living 

 protoplasm and we may ask, is there any experimental evidence 

 to show that this condition may be overcome by natural means ? 



It has been generally assumed by biologists that conjugation 

 brings about rejuvenescence in the conjugating individuals, and 

 so imparts to the ex-conjugants and to their immediate descend- 

 ants a high potential of vigor. During the process of conjugation 



