REJUVENESCENCE IN UROLEPTUS MOBILIS 125 



times for identical calendar periods, we can tell whether fluctu- 

 ations in vitality are due to conditions of the environment or to 

 inherent vitality of the protoplasm. If due to environmental 

 conditions, a correction may be made which will indicate, approx- 

 imately, the comparative vitality, had the environmental con- 

 ditions been normal. 



Again, if we plot the curves for different series and start them 

 all from the same ordinate, we have a means of measuring the 

 vitality of different series at similar stages of the life cycle. Such 

 curves enable us to find the cyclical incidence of important 

 phases of the life history, such as conjugation and encystment, 

 and to establish the limits within which they occur. 



These methods have been used in working out the results 

 described in the following pages. In maintaining the cultures 

 and keeping up the records, as well as in working up the statistical 

 data, I have been fortunate in having the assistance from time to 

 time of Miss Mabel L. Hedge and of my colleague Prof. Louise 

 H. Gregory, whose helpful interest I gratefully acknowledge. 



GENERAL HISTORY OF THE CULTURES 



The fact should be emphasized at the outset that these ex- 

 periments deal, in the main, with one bit of protoplasm which 

 emerged from the processes of conjugation on November 16, 

 1917, reproduced abundantly by division, underwent paedoga- 

 mous conjugation repeatedly, underwent encystment, and is still 

 living with a vigor equal to that at the beginning. Some of this 

 protoplasm has been maintained in isolation cultures whereby 

 conjugation has been prevented and in which encystment does 

 not occur. Such protoplasm invariably dies, the phenomenon of 

 natural death being the last stage of a decreasing vitality — a 

 decrease which begins to show early in the life cycle. Other 

 parts of this protoplasm have been allowed to conjugate among 

 themselves, the subsequent isolation cultures showing the effects 

 of such conjugation upon such protoplasm. Still other parts of 

 this protoplasm have been allowed to encyst and to undergo 

 processes of asexual reorganization within such cysts, and the 

 effects of such reorganization have been ascertained. 



