A TYPICAL LIFE CYCLE 



109 



became vacuolated instead of granulated. There is no doubt, from 

 these daily observations on the same organisms, that there is a change 

 in physiological activity, which cannot be interpreted as due to the 

 difference in the relative sizes of nucleus and cytoplasm, but must be 

 traced to some more deeply lying cause. 



After two similar periods of depression had been successfully offset 

 by artificial means, a fourth and final periotl. in which the protoplasmic 

 structm-es were cpiite dilferent from previous conditions, carried off 

 the last generation of the race, 742tl generation (see p. 129). 



ri.i. 39 



Paramecium aurelia :il iieriod nf depression, sliowiiiK {at left) the dense granular condition 

 of tlie jirotoplasm, which, if Tiot relie\-cd artificially, in\ariabl.v ends in death. The central 

 and riffht hand figiu'es show the elTccts of such artificial relief in the vicinity of the nucleus, 

 while the extremities are still dense. 



While these initial experiments would seem to indicate a certain 

 normal length of life (ajiproximately 200 to 800 generations), it does 

 not follow that all paramecia have the same endowment. Different 

 races of paramccium, like tlifferent human individuals, vary in the 

 initial potential of vitality, and are capable of living for different 

 lengths of time upon the same medium. Thus, other cultures of Para- 

 mecium, carried on at the same time as those described, yielded 376 

 and 379 generations before evidences of depression set in. A con- 

 stantly changing medium, furthermore, may tend to oft'set the cumula- 

 tive physiological weakness and so to prolong the life of the race. 

 Such an experiment on paramecium has recently been carried out by 

 Woodruff ('OS), who, instead of constant hay infusion, used infusions 

 of leaves, grass, etc., from natural pond water, frequently changing 



