38o SENESCENCE AND REJUVENESCENCE 



differences can at most be merely those between physiologically older 

 and younger individuals both of which have attained the adult form, 

 and in organisms as simple as the ciliates would be more readily 

 distinguishable physiologically than morphologically. But so far 

 as I am aware, this point has not been considered by most students 

 of protozoa. One author, Prowazek ('lo), has stated that when 

 cultures of Colpidium are prevented from dividing by insufficient 

 nutrition, they rapidly become old. In his cultures, however, the 

 animals were evidently starved, for they underwent reduction in 

 size to a considerable extent and their susceptibiHty to atropin 

 underwent a marked increase. These changes in size and suscepti- 

 bility suggest that these cultures were undergoing reduction and 

 increase in rate of metabolism in consequence of partial starvation 

 (cf. chap, vii) instead of undergoing senescence. Nutrition seems 

 to have been insufficient in this case to permit senescence to occur; 

 for that the food should have been at least sufficient to prevent 

 decrease in size. 



But the important point is that those conditions which favor a 

 progressive senescence are the conditions which favor conjugation. 

 The facts from this point of view suggest simply that under con- 

 ditions which favor rapid agamic reproduction the animals have 

 no opportunity to attain maturity because of the frequent recon- 

 stitution and rejuvenescence. When agamic reproduction is 

 retarded or inhibited, maturity is very soon attained and conju- 

 gation occurs. 



The occurrence of endomixis indicates, as I have pointed out, 

 that the meganucleus undergoes senescence in spite of agamic 

 reproduction. If, at the time when the meganucleus is approach- 

 ing death, the cytoplasm is physiologically young and in good 

 metabolic condition, then apparently endomixis and recovery with- 

 out conjugation occur, but if, at this time, the cytoplasm is also in 

 a condition of advanced physiological senescence, then probably 

 the physiological conditions for conjugation are present, and if con- 

 jugation is impossible, death may result. According to this view, 

 endomixis results from progressive senescence of a single specialized 

 organ, the meganucleus, and conjugation from the senescence of 

 both meganucleus and cytoplasm. 



