422 BIOLOGY: GENERAL AND MEDICAL 



In these cases the age of the animal experimented upon 

 plays an important part in the success of the experiment, 

 better results being obtained with larval than with adult 

 organisms. 



Earth-worms cut apart and then stitched together 

 readily unite, and by cutting parts from several worms 

 and stitching them together unusually long worms can be 

 made, or by cutting off a few of the anterior segments 

 and stitching them to a few segments cut from the tail 

 unusually short worms can be produced. This reminds 

 us that the replacement of the excised part inhibits the 

 process of regeneration. When the leg of a salaman- 

 der is cut off, a new leg regenerates as has been shown; 

 but if the removed leg be replaced and stitched to the 

 stump, it grows fast and no new leg develops. Similarly, 

 if the tail of a tadpole be amputated, a new tail regen- 

 erates, but if the amputated tail be carefully replaced 

 it grows fast and no regeneration occurs. If in replac- 

 ing the tail the work be done carelessly so that the 

 coaptation of tail and body be imperfect and a part of 

 the stump left unprotected, the tail may grow fast, 

 but from the uncovered part of the stump a new tail 

 grows, so that the animal becomes provided with two 

 such members. 



When the anterior twenty segments and the posterior 

 twenty segments of an earth-worm are amputated, the 

 former regenerates all the necessary remaining posterior 

 segments; the latter all the necessary anterior segments, 

 and the middle portion, all necessary anterior and pos- 

 terior segments, so that three complete worms eventually 

 form. But if the anterior twenty segments are stitched 

 to the posterior twenty segments, the two portions 

 grow together, no intermediate segments are regenerated, 

 and a short worm is formed and remains short. 



Many experiments with interesting results along this 

 line were made by Joest, who found that the "satisfaction 

 of physiological necessity" did not seem to be the key to 

 the situation, seeing that there might be two sets of 



