492 BIOLOGY OF THE PROTOZOA 



the former, series after series have been followed from high initial 

 vitality after fertilization until death occurred. In more than 

 one hundred and twenty such series the history has been the same 

 but with variations in time and in number of generations well illus- 

 trated by the series selected from the records for different years 

 and shown in Fig. 198. The last individuals of such series may 

 show a remarkable tenacity in vitality but without the power to 

 reproduce. Of 283 such "last individuals" 1 lived more than ninety 

 days; 2 lived more than sixty days; 7 more than forty days; 15 

 more than thirty days; 26 more than twenty days; 88 more than 

 twelve days; while the remainder lived from one to ten days. In 

 all of these cases the old individuals were transferred daily to 

 fresh medium from the same source as that in which other, younger, 

 individuals w^ere dividing from one to three times per day. In 

 most of the old specimens apart from the reduced division rates, 

 there is little e^'i(lence of physiological weakness. They move with 

 the usual vigor and apparently maintain an equilibrium between 

 income and outgo for many days. This condition is the outcome 

 of a gradually waning vitality which in turn may be due to a slowly 

 increasing stability of substances in the protoplasmic organization, 

 or as Robertson (1921) suggests, to accumulation of substances 

 which can no longer be discharged from the cell. This I interpret 

 as evidence of old age difl'erentiation with the same fatal termina- 

 tion as that which follows highly flifl'erentiated gametes which fail 

 to unite in fertilization. 



In many organisms this physiological deterioration is accompanied 

 and manifested by structural degenerations. Maupas (1888) 

 noted the loss of micronuclei in old age ciliates as well as other 

 degenerations involving the motile organs (Fig. 197). The observa- 

 tions have been fully confirmed with VroJepiuft mohilis particularly 

 in regard to the loss of micronuclei, but also noticeable in the extreme 

 vacuolization of the protoplasm (Fig. 7, p. 28). In Paramecium 

 caudatuvi and in individuals which have not conjugated for a long 

 period, old individuals are characterized by hypertroi)hy of the 

 micronucleus and by the loss of trichocysts in the cortex. 



Still another outcome of the physiological weakness is the tendency 

 to divide abnormally thus leading to monster formation. This 

 has been typical of all old age cultures which haAe come under my 

 observation. Such monsters are strikingly like those formed as 

 a result of cutting Paramecium (see supra p. 486) but they never 

 grow into large amorphous masses of protoplasm which frequently 

 develop from mutilated Paramecium individuals (Fig. 205). 



The old age phenomena discussed a])ove all iuAolve a physiological 

 weakness or reduced \'itidity which may well l)e traced back to 

 increasing stability of protoplasmic substances, and lead to a 

 break-down in the protoplasmic organization. A fourth type has 



