106 PROTOPLASMIC AGE OF PROTOZOA 



continued existence, they apparently offer some justification for the 

 older view that protozoa are practically deathless, so far as old age 

 is concerned. 



The matter of physical immortality can be easily tested, however. 

 After a little practice, a single cell of parameeium can be isolated and 

 fed on the bacteria which develop in a previously sterilized hay infu- 

 sion made by boiling small pieces of hay in water. The organism is 

 placed in a small chamber filled with the hay infusion and made by 

 supporting a coverglass on pieces of glass. When it divides, which it 

 will do within twenty-four hours, the daughter cells can be similarly 

 isolated and fed on freshlv made hav infusion, and in this wav the 



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vitality of that originally minute bit of protoplasm can be watched 

 day after day and generation after generation of cell divisions, until 

 natural death from old age ensues. The writer successfully followed 

 the life history of such a culture of parameeium from an initial cell to 

 protoplasmic death from old age, giving fresh food medium and 

 isolating the single cells day after day and generation after generation 

 for a period of twenty-three months and 742 generations. The obser- 

 vations made during such a study deal with living protoplasm that 

 is growing old more rapidly than in nature, and with the ageing 

 process in an organism endowed with an initial potential of vitality. 



A parameeium which is thus followed from generation to generation 

 shows surprisingly regular variations in vitality. Some of the more 

 minute variations are due to temperature changes, a warm day, for 

 example, increasing, a cold day diminishing, their vigor. In the 

 laboratory, however, such variations may be overlooked, for the 

 changes in temperature from day to day are of minor importance. 

 After much experimenting, a measure of vitality was finally found 

 which made it possible to compare the activity of the physiological 

 processes from time to time. This measure was represented in the 

 form of a curve, the points upon it being obtained by averaging the 

 number of divisions made by all of the organisms under observation 

 in periods of ten days, each average giving the ordinate for one period; 

 the abscissas represent the arbitrary ten-day periods (see Fig. 38). 



Such a curve, representing the vitality of the parameeium proto- 

 . plasm, shows that in a period of six months under cultivation, if the 

 organisms are fed upon the same diet of hay infusion, there is a gradual 

 exhaustion of vitality, the curve falling from an average of about 

 twelve divisions in ten days in February to an average of one division 

 in ten days in July. As the curve shows, the average number of cell 

 divisions sinks more or less regularly during the six months, but 

 undergoes periodic rises and falls, until at the end of that time the 

 organisms are unable to digest and assimilate the bacterial food and 

 the cells begin to die, the minute cellular corpses being abundant at 

 such a period. 



