AGAMIC REPRODUCTION AND REJUVENESCENCE 145 



plasmic than nuclear. That the age cycle of certain flatworms may 

 be altered to a very considerable extent by experimental nutritive 

 and other conditions will be shown in chap. vii. Moreover, the 

 different behavior of different races as regards conjugation' suggests 

 that internal as well as external factors will be found to play a part 

 in determining the periodicity. 



But whatever the differences resulting from race or environ- 

 mental conditions, the occurrence in the ciliates of some degree of 

 senescence in each generation and some degree of rejuvenescence 

 in each agamic reproduction and the occurrence of progressive 

 senescence in the meganucleus ending in its death and replacement 

 by a new, young organ demonstrate that these unicellular animals 

 are not fundamentally different from multicellular forms. They 

 are not, as Weismann ('82) believed, immortal because they do not 

 grow old, but simply as other organisms are, because they repro- 

 duce and undergo reconstitution during reproduction and because 

 old organs die and are replaced by young. 



AGAMIC REPRODUCTION AND REJUVENESCENCE IN COELENTERATES 



Among the coelenterates only the fresh-water hydra and one 

 species of the colonial hydroids have been tested by the suscepti- 

 bility method. In hydra agamic reproduction is a process of 

 budding. In Hydra j'usca the bud arises near the junction of the 

 thicker body with the more slender stalk, and in its earlier stages is 

 merely a rounded outgrowth including both ectodermal and cnto- 

 dermal layers of the body- wall (Fig. 48). Cell division and growth 

 occur rapidly in it, it elongates, and in the course of a few days 

 tentacles and a mouth begin to develop at its distal end (Fig. 49). 

 Meanwhile the region of attachment to the parent body gradually 

 undergoes constriction, until finally the new, small animal separates 

 from the parent, falls to the bottom, attaches itself, and begins 

 to lead an active life. In this process a portion of the body-wall of 

 the parent has undergone reconstitution into a new, independent 

 individual. 



A comparison of the susceptibility to cyanide of small animals 

 newly developed in this way with the larger parent shows that the 



"Jennings, '10, '13; Calkins and Gregory, '13. 



