MORE COMPLICATED LIFE CYCLES 133 



micronucleus, which has but httle part to play apparently in the 

 ordinary functions of vegetative life, now appeared enlarged and 

 vesicular, and entirely difi'erent in structure and size from the micro- 

 nucleus of the ordinary paramecium. The protoplasm was not granu- 

 lar nor chemically stable, and was apparently as active as ever. Still 

 the organisms died, and death was not due to infection or disease. 

 Something in the cells that had been operative before had given out, 

 and the only part of the cell which had not responded to treatment was 

 the micronucleus. Here, then, was a pathological condition which 

 could not be met, and the organisms died. 



« Fig. 55 



A "monster" formed by incomplete division of Parameciunr aurelia as an indication of tlie 

 exhaustion of division energy. (After Calltins.) 



Was it death from old age that carried off the race under obser- 

 vation? There seems to be no other alternative to consider, and by 

 old age we mean the wearing out of an organ and the cessation of a 

 function. If old age may be thus defined in a simple organism like 

 Paramecium, it follows that three times previously had the race been 

 weakened by old age, since the organisms were unable to digest and 

 assimilate food. As soon as this power was restored by artificial means, 

 old age was overcome and cell division was resumed. The cells would 

 have died without any doubt had they not been stimulated, so that we 

 are justified, as I believe, in speaking of this condition of paramecium 

 as "physiological" old age, which leads to physiological death through 

 the cessation of one or more of the vegetative functions. It is 

 obviously death from a different cause that carried off the last cells of 

 the race, and since the ordinary vegetative functions were apparently 

 in perfect working condition at this final period, it follows that the cause 

 of death must be looked for in the cessation of some other than the 

 ordinary vegetative activities. The history of the micronucleus in 

 conjugation (see next chapter) shows that this is the organ of the 

 Paramecium cell endowed with the characteristics of the race; in other 

 words, that it alone of all the structures of the cell, must contain the 



