Ii8 SENESCENCE AND REJUVENESCENCE 



from different body levels and are correlated with differences in 

 the process of reconstitution. 



If pieces smaller than these are taken, the increase in suscepti- 

 bility is greater and the animals attain the condition of still younger 

 sexually produced forms. Evidently these experimental repro- 

 ductions, while they do not carry the organism back to the 

 beginning of development, do carry it back to the physiological 

 condition characteristic of the sexually produced, growing animal 

 of the same size. Experimental reproduction is apparently in this 

 species just as efficient a means of producing physiologically young 

 animals as sexual reproduction. 



REPEATED RECONSTITUTION 



It has been shown in preceding sections that the animals 

 produced by reconstitution are physiologically younger than the 

 animals from which the pieces are taken, and moreover that they 

 are about as young as sexually produced animals of the same size. 

 If this is the case, it should be possible to breed animals indefinitely 

 by means of this process of experimental reproduction. On the 

 other hand, the animal rejuvenated by reconstitution may differ in 

 some way from the sexually produced animal, but so slightly that 

 the difference does not become apparent in a single generation, 

 but requires several or many generations of breeding by experi- 

 mental reproduction to become distinguishable. Thus far two 

 attempts at reconstitutional breeding have been made, both of 

 which were terminated by accident, but one of them continued 

 long enough to throw at least some light on the question. 



The breeding stock for these experiments was obtained as 

 follows: Large individuals of the same size, which had been kept 

 under uniform conditions, were selected, and from each of these 

 a piece of a certain size and from a certain region of the body was 

 taken. These pieces were allowed to undergo reconstitution and 

 after this was completed were fed until they attained approxi- 

 mately the original size. Then from each a piece, including the 

 same region of the body, was taken; these were again allowed to 

 develop, were fed, and so on. In one of these breeding experiments 

 the piece used in each generation was the anterior fifth of the body, 



