no SENESCENCE AND REJUVENESCENCE 



physiologically, than the old tissues of the rest of the piece. As 

 the new tissue differentiates, however, this difference in suscepti- 

 bility between it and the old parts gradually disappears, for the 

 new tissue gradually grows old and its rate of metabolism decreases, 

 while the old tissue gradually undergoes reconstitutional changes 

 which involve the atrophy and disappearance of some parts and the 

 formation of others by cell division and growth, and besides this 

 the tissues of the piece, particularly the old tissues with their lower 

 rate of metabolism, are being used up as a source of nutrition for 

 the developing organism. In other words, the new embryonic 

 tissue formed at the cut surfaces gradually becomes old after its 

 formation, while other parts of the piece gradually become young 

 by reduction and reorganization, until a dynamic equilibrium is 

 established in the rate of metabolism in the different parts, after 

 which the animal, if fed, undergoes senescence as a whole. 



With various other organisms which show a high capacity for 

 reconstitution similar results have been obtained. In various other 

 species of flatworms, so far as tested, in Hydra and in the hydroid 

 Corymorpha, the animals resulting from the reconstitution of 

 pieces show a higher rate of metabolism than the animals from which 

 the pieces were taken. Miss Hyman has found that this is also 

 true for animals developed from pieces of Lumbriculus and other 

 fresh- water oligochete annelids. 



Animals produced in this way are also younger in other respects 

 than those from which the pieces came. They grow more rapidly 

 and are capable of repeating the developmental history from the 

 stage which they represent onward. There can be no doubt that 

 the process of reconstitution brings about in some way a greater 

 or less degree of rejuvenescence in these relatively simple animals, 

 and that the degree of rejuvenescence is in general proportional to 

 the degree of reorganization in the process of reconstitution of the 

 piece into a whole. 



THE INCREASE IN SUSCEPTIBILITY IN RELATION TO THE DEGREE 



OF RECONSTITUTION 



The reconstitutional capacity of pieces of Planaria dorotocephala, 

 as of other species, is limited. Pieces below a certain size limit, 



