1 68 SENESCENCE AND REJUVENESCENCE 



animals. Susceptibility determinations have not been made, since 

 the stock is not large and is gradually depleted by occasional acci- 

 dental losses in changing water. However, there is every reason to 

 believe that the animals are as young physiologically as their size 

 would lead one to suspect, and they have shown no indications of 

 the changes in color, cessation of feeding, and decrease in motor 

 activity characteristic of old worms. 



While the animals of this insufficiently fed stock have remained 

 at essentially the same physiological age during almost three years, 

 another portion of the same original stock which emerged from 

 cysts in the laboratory at the same time, but which has been fed 

 often enough to permit rapid growth, has passed through thirteen 

 asexual generations. A comparison of these two stocks leaves no 

 doubt as to the effect of partial starvation in inhibiting senescence 

 and the changes accompanying it. In these animals the length 

 of life or of the developmental period is not measured by time, but 

 by rapidity of growth. With abundant food this species may pass 

 through its whole life history, from the stage of emergence from a 

 cyst to fragmentation and encystment, in three or four weeks, but 

 when growth is prevented by loss of food, it may continue active 

 and young for at least three years, as the foregoing experiment has 

 demonstrated, and doubtless for a much longer period. It is of 

 course possible that continuation of the experiment during a suffi- 

 ciently long time might show that a slow process of senescence was 

 occurring in spite of the absence of growth. Only such continua- 

 tion can determine whether this will be the case or not. But the 

 fact remains that senescence can be retarded or inhibited for a 

 length of time, which, compared with the length of the active life 

 in nature, is very long in the present case about thirty-six times 

 as long, and eighteen times as long as the average length of a genera- 

 tion in the laboratory. 



Similar experiments with Planaria dorotocephala have been 

 carried sufficiently far to show that this species also can be kept 

 in approximately the same physiological condition for some months. 

 As long as the animals do not receive food enough to permit growth, 

 there are no indications of senescence, but when growth occurs the 

 susceptibility begins to decrease. 



