VITALITY 269 



made during the experiments it is possible to work out the inci- 

 dence of encystment and of conjugation for each series. Fig. 137 

 shows the vitality curve of ten different series. The periods of the 

 first encystments observed and the last encystments in the different 

 series are connected by vertical lines. The first appearance of con- 

 jugation is indicated in the same manner but with double lines. 

 In some series it happens that both encystments and conjugations 

 occur in the same container but tests of the same series made later 

 give only conjugations. With Uroleptus at least it appears, there- 

 fore, that encystment is a characteristic phenomenon of young 

 organisms comparable with the Dauersporen of phytoflagellates, 

 and lower plants generally, after fertilization; and that the power 

 to form reorganization cysts disappears with the advent of maturity. 

 It is highly desirable to have similar data for other types of ciliates 

 and to determine whether or not endomixis occurs in each case. 



(b.) Cyclical Differentiations Peculiar to Old Age. Toward the 

 end of the life cycle even more characteristic differentiations occur 

 than at the outset. In many cases these are coincident with the 

 fertilization phenomena and will be discussed in connection with 

 differentiations at maturity. The most significant of these age 

 differentiations are: (1) Greatly reduced vitality; (2) structural 

 degeneration; (3) abnormal divisions leading to monster formation; 

 (4) special structures appearing at no other time in the life cycle. 



The best evidence of reduced vitality toward the end of the 

 cycle is afforded by Uroleptus mobilis and Spathidium spathula. In 

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

 vitality after fertilization until death occurred. In more than 

 one hundred and forty 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 of different years 

 and shown in Fig. 131. 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 were dividing from one to three times per day. In 

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

 there is little evidence 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 



