THE RECONSTITUTION OF ISOLATED PIECES 119 



including the old head. In such pieces the old head remains from 

 one generation to another and new tissue appears only at the 

 posterior end; consequently the amount of reorganization is less 

 than in pieces which form a new head or in pieces from the posterior 

 region of the body. Moreover, the head-region is less capable of 

 reorganization than other parts of the body. If a progressive 

 senescence occurs from generation to generation in spite of recon- 

 stitution in each generation, it should become more distinct or 

 appear earlier in such pieces than in those where the reconstitu- 

 tional changes are more extensive. 



In the course of a year and a half the animals passed through 

 thirteen experimental generations without any indications of 

 senescence or depression of any sort. During the growth of the 

 thirteenth generation, however, most of the stock was killed by 

 high temperature and the remaining animals never regained good 

 condition, but died in the course of the next few generations. The 

 worms that remained alive in each generation grew more or less 

 normally, and the breeding was continued with these. In the six- 

 teenth generation only eight worms remained alive, and in order to 

 determine whether more extensive reconstitutional change would 

 bring the animals back to their original condition, the old heads 

 were removed and each animal was cut into several pieces. Some of 

 these pieces produced complete animals, but deaths continued to 

 occur among these, and some of the pieces died without reconstitu- 

 tion. The living animals were again cut into pieces after growth, 

 and this was repeated to the nineteenth generation in which the 

 last of the stock died without recovery. 



In another stock pieces from the middle region of the body were 

 used for each generation. In the fifth generation this stock was 

 subjected to high temperature at the same time as the preceding, 

 and most of the animals died. Those that remained alive gradually 

 died during the following generations, until in the tenth genera- 

 tion all were dead. 



The results of these two breeding experiments are of value only 

 as far as they go. The first does show, however, that the animals 

 can be bred by experimental reproduction without loss of vigor 

 for at least thirteen generations, even when the old head is 



