126 



SENESCENCE AND REJUVENESCENCE 



34 



33 



FIGS. 32-34. Reconstitution after fission in 

 Planar ia dorotocephala: Fig. 32, animal before 

 fission; jf, fission-plane, a, anterior, b, posterior 

 zooid; Fig. 33, reconstitution of posterior zooid; 

 Fig. 34, reconstitution of anteriorzooid. 



natural process of agamic 

 reproduction resembles so 

 closely the process of 

 reconstitution the occur- 

 rence of some degree of 

 rejuvenescence is to be 

 expected in agamic repro- 

 duction. 



It has already been 

 shown in Fig. 3 (p. 80) 

 and in Fig. 13 (p. 100) that 

 individuals of P. doroto- 

 cephala of small size and 

 young in appearance, but 

 which supposedly arose 

 agamically, are physiologi- 

 cally much younger as re- 

 gards their susceptibility 

 than the large, apparently 

 old animals. But in order 

 to obtain conclusive evi- 

 dence upon this point it is 

 necessary to compare ani- 

 mals which are known to 

 have arisen by fission 

 under controlled con- 

 ditions with animals like 

 those in which the fission 

 occurred. 



This comparison has 

 been made repeatedly and 

 the result confirms expec- 

 tation. The small animal 

 which develops from the 

 separated posterior region 

 of the parent animal is 

 physiologically much 



