412 Louise Hoyt Gregory 



all cases, the body is shown to be plastic and unstable. Attempts 

 were made to try the power of regeneration in the centrifuged 

 individuals, but this was not successful as the organisms were in 

 too weak a condition, and went to pieces on being cut. 



VIII GENERAL CONSIDERATIONS 



Artificial Rejuvenesence 



Like the division rate of Paramecium and of Oxytricha, that 

 of Tillina shows the same rhythmic variabihty, representing peri- 

 odic variations in the vitality of the protoplasm. Unlike Parame- 

 cium and Oxytricha, the division rate of Tillina does not indicate 

 as definite a response to treatment with salts. Such substances, 

 apparently successful in other forms, seem to have been effective 

 only in raising the vitality slightly above the normal, and increas- 

 ing it sufficiently to carry the protoplasm through periods of weak- 

 ness, and the question arises, has the protoplasm been rejuve- 

 nated ? According to the definition of Woodruff: "a cycle is a 

 periodic rise and fall in the fission rate, extending over a varied 

 number of rhythms, and ending in the extinction of the race unless 

 it is 'rejuvenated' by conjugation or a changed environment." 

 Following the definition, the first impression would be that the 

 25th period of the Tillina curve marks the end of the first cycle, 

 the stimulation of the KoHPO^ of two periods previous, affording 

 the changed environment the influence of which carried one cul- 

 ture through the period of lowered vitality. If this is true, a 

 second cycle ends at the 35th period, and the third at the 40th 

 period. In all, then, there would be three cycles, the first lasting 

 eight months, the second and third three months each.. A careful 

 study of the history of TiUina has convinced me that the curve 

 of vitality represents one cycle only. As has already been men- 

 tioned, there was no markedly high period of activity resulting 

 from a stimulation as in the case of Paramecium and Oxytricha. 

 The slight impetus that was given was only temporary in its effect, 

 and after a comparatively short time a re-stimulation was neces- 

 sary to carry the individual along at even a normal rate. From 



