120 SENESCENCE AND REJUVENESCENCE 



continuously present. The first stock was subjected to high 

 temperature in the thirteenth generation, the second in the fifth 

 generation, but in both the result was the same, in that most of 

 the stock was killed and the survivors failed to recover after several 

 months. There can be little doubt that the high temperature 

 rather than the physiological condition of the animals was respon- 

 sible in one way or another for the death of both stocks. 



As a matter of fact, however, the question which these experi- 

 ments attempted to answer is answered by reproduction in nature 

 in Planaria dorotocephala and P. velata. It will be shown in the 

 following chapter that the process of agamic reproduction in these 

 forms is not essentially different in any way from the process 

 of reconstitution of pieces, and this is the only method of reproduc- 

 tion which has been observed in these two species under natural 

 conditions. 



The results obtained by another method of experiment are of 

 interest in this connection. This is essentially breeding by experi- 

 mental reproduction without food. Pieces from large, old animals 

 are allowed to undergo reconstitution; then, without feeding, pieces 

 are taken from these animals, and so on. Here of course each 

 genera Jon is smaller than the preceding, and the experiment is 

 finally brought to an end by the advancing starvation of the animals 

 and the failure of the minute pieces to undergo reconstitution. But 

 susceptibility tests show that the susceptibility increases with such 

 reconstitutions, and in Planaria maculata, where sexually produced 

 animals are available for comparison, the animals after a few genera- 

 tions of reconstitution without food show a susceptibility equal to 

 that of animals just hatched from the egg capsule. Their rate of 

 metabolism has increased in consequence of the successive recon- 

 stitutions and the absence of food until it equals that of very 

 young sexually produced animals. If fed after such a series of 

 reconstitutions, they grow and are indistinguishable from the 

 animals hatched from eggs. 



In short, by successive reconstitutions alternating with feeding 

 and growth, the animals may be brought back to essentially the 

 same stage in the age cycle in each successive generation, and by 

 successive reconstitutions without feeding and growth they may be 



