io8 



FIG. 1 8. Body-outline 

 of Planar ia dorotocephala, 

 indicating levels of section. 



SENESCENCE AND REJUVENESCENCE 



cut. These two lots of pieces were allowed 

 to develop into new animals. A third part 

 of the stock consisting of uninjured worms 

 was kept under the same conditions as a 

 control and since the pieces do not feed 

 during the process of reconstitution, this 

 third lot was not fed. During the recon- 

 stitution of the pieces several comparative 

 tests were made of their susceptibility, and 

 of that of the uninjured animals, to 

 cyanide. The results of one of these tests 

 made sixteen days after the pieces were 

 cut from the parent bodies is given in 

 Fig. 19. Both the pieces and the whole 

 animals had been without food during this 

 time, but the effects of sixteen days' 

 starvation are not very great as regards 

 susceptibility. During these sixteen days 

 the pieces had become fully developed 

 animals, the longer being seven to eight 

 millimeters, the shorter, five millimeters in 

 length. In Fig. 19 the curve ab shows the 

 susceptibility of ten animals developed 

 from the shorter pieces, the curve cd the 

 susceptibility of ten animals from the 

 longer pieces, and the curve ef the suscepti- 

 bility of uninjured animals the same size 

 as those from which the pieces were taken. 

 It is evident at once from the figure that 

 the susceptibility of the pieces which have 

 undergone reconstitution to whole animals 

 is very considerably greater than that of 

 the uninjured animals like those from 

 which these pieces came, and that further 

 the susceptibility of the animals which 



develop from the shorter pieces is greater than that of those from 

 the longer. The results of all other similar tests of susceptibility 



