CONCLUSIONS FROM EXPERIMENTS 181 



older and their rate decreases as they grow and differentiate. At 

 the same time, the remaining parts of the piece are drawn upon 

 as a source of energy for the growth of the new parts, and in con- 

 sequence they undergo reduction and their rate of metabolism 

 rises: in fact, they become younger. Sooner or later a condition is 

 attained in which the young, new parts can no longer grow at the 

 expense of the old parts because the rate of metabolism in the former 

 is declining while that in the latter is increasing. When this stage 

 is attained reconstitutional changes can proceed no farther. If 

 the animal is fed at this stage it grows essentially like any other 

 animal, and if not fed it undergoes reduction like any other starved 

 animal. At the time equilibrium is attained the rate of metabolism 

 in general will vary with the size of the piece and the degree of 

 reconstitution. The smaller the piece and the greater the amount 

 of reconstitutional change, the higher the rate at which this equi- 

 librium is reached, and so the younger the animal becomes during 

 reconstitution. 



As already noted, the cases of agamic reproduction examined 

 in chap, vi do not differ fundamentally from the experimental 

 reproductions or reconstitutions following the physical isolation 

 of pieces, and we should expect that if rejuvenescence occurs in 

 the one case it would in the other. Whether a piece develops into 

 a new whole as the result of artificial isolation by section or other 

 means, or of physiological isolation by conditions arising in the 

 organism in nature, the result is essentially the same. In one 

 respect, however, there is a difference of degree: in many cases of 

 budding, fission, etc., the new developing individual remains in 

 organic continuity with the parent until its development is ad- 

 vanced or completed and so is supplied with nutritive material. 

 In such cases, as for example, in hydra, the new individual, instead 

 of undergoing reduction, grows throughout its development, and 

 the degree of rejuvenescence is much less marked than in those 

 cases where the tissues of the developing piece or region are the 

 source of energy. Here the dedifferentiation of cells, or the sub- 

 stitution of less differentiated younger cells for those previously 

 existing, are the chief factors in rejuvenescence, although appar- 

 ently some degree of metabolic equilibration does occur in the old 



