128 



SENESCENCE AND REJUVENESCENCE 



maculata the animals developed from these pieces are about as 

 young physiologically as sexually produced animals of the same 

 size. In P. dorotocephala, where sexually produced animals are 

 not available for comparison, the degree of increase in suscepti- 

 biUty over that of the parent animals is about the same as in F. 

 maculata. Since these results are so completely in agreement, 

 both with expectation and with the results obtained from arti- 

 ficially isolated pieces, experimental records are unnecessary. 



Stages J- ^ ^ 



Hours 234567 



Fig. 35. — Susceptibility of Planar ia dorotocephala to KCN o.ooi mol.: ab, 

 anterior fission-pieces after reconstitution; cd, entire animals before fission. 



With respect to the anterior fission-piece, however, it is a matter 

 of some interest to demonstrate that the reconstitutional changes 

 occurring in the posterior region of so large a piece as this do alter 

 the physiological condition of the whole piece, including even the 

 head-region. For this reason the record of one susceptibility test 

 of these anterior pieces is given in Fig. 35. For this experiment 

 worms ten to twelve millimeters in length were induced to undergo 

 fission and the anterior fission-pieces were kept without food for 

 twelve days. Another lot of worms of the same size and in the 



