136 SENESCENCE AND REJUVENESCENCE 



naturally occurs may be induced by strong stimulation, and in such 

 cases development and the increase in susceptibility are usually 

 somewhat accelerated. 



From these facts we must conclude that in Stenostomum as in 

 Planaria the reconstitution of a given region of the body into a 

 new individual is accompanied by some degree of physiological 

 rejuvenescence. Without doubt the age differences in suscepti- 

 bility between the developing young zooids and the fully developed, 

 relatively, old anterior zooid of the chain are obscured to some extent 

 by the much greater motor activity of the latter, but the fact that 

 sooner or later the young zooids become more susceptible than this 

 older zooid indicates that rejuvenescence does occur. 



In various species of aquatic oligochete annelids agamic repro- 

 duction occurs in much the same manner as in Stenostomum. In 

 the course of investigations as yet unpublished Miss Hyman has 

 found that these animals, like the flatworms, undergo a greater or 

 less degree of physiological rejuvenescence in connection with 

 agamic reproduction. 



THE RELATION BETWEEN AGAMIC REPRODUCTION AND REJUVENES- 

 CENCE IN PROTOZOA 



The question whether the protozoa undergo senescence or not is 

 of considerable interest at present. The generally accepted view 

 based on the researches of Maupas ('88, '89) that conjugation in 

 the ciliate infusoria terminates an invariable process of race senes- 

 cence and brings about rejuvenescence requires some modification 

 in the light of recent researches. Woodruff has bred a race of 

 Paramecium through nearly five thousand generations without 

 conjugation and without loss of vigor. 1 This number of gen- 

 erations is so large that we are justified in maintaining that 

 for the race of Paramecium used, and under the conditions of 

 experiment, conjugation is not an essential feature of the life 

 cycle. On the other hand, various investigators 2 have shown 



1 Woodruff, '08, '09, 'na, '133, '13^, '14; Woodruff and Erdmann, '14. In 

 these and other papers the author records the progress of the agamic breeding. 



2 Among these may be mentioned Calkins, '020, '026, '04; Enriques, '03, '07, 

 '08; Woodruff (see note i); Jennings, '10, '13; Baitsell, '12, '14; Zweibaum, '12; 

 Calkins and Gregory, '13. 



