ii6 



SENESCENCE AND REJUVENESCENCE 



(D ® 



THE SUSCEPTIBILITY OF ANIMALS RESULTING FROM EXPERIMENTAL 

 REPRODUCTION AND SEXUALLY PRODUCED ANIMALS 



The belief that the germ cell is the source of youth and that the 

 old organism cannot become young has been so widely current 

 among biologists that it is of some interest to 

 determine whether the physiological condition 

 of the animal resulting from reconstitution 

 approaches that of the sexually produced yoimg 

 animal. Planaria dorotocephala is not available 

 for such experiments, since it does not repro- 

 duce sexually under ordinary conditions, con- 

 sequently another species, P. maculata, has been 

 used in which the young produced from eggs 

 can readily be obtained. 



In experiments of this kind pieces were cut 

 from old, sexually mature animals and allowed 

 to undergo reconstitution; after reconstitution 

 their susceptibility was compared with that of 

 sexually produced young of the same size. In 

 the particular experiment of which the results 

 are given in Fig. 27 below, two lots of pieces 

 {a and b, Fig. 26) were cut from old, sexually 

 mature worms twelve millimeters in length. 

 These pieces were left for ten days under uni- 

 form conditions, at the end of which time they 

 had become normal animals five to six milli- 

 meters long. They were then fed, and two days 

 later their susceptibility was compared both 

 with that of old, sexually mature worms lik e 

 those from which the pieces were taken and also 

 with that of young, growing worms five to six 

 milHmeters long, which had been hatched from 

 eggs in the laboratory. 



Fig. 27 shows the susceptibilities to KCN 



o.ooi mol. of ten old, sexually mature worms 



growing worms hatched from eggs {ah, long 



developed from the a-pieces {ah, short 



Fig. 26. — Body- 

 outline of Planaria 

 maculata, indicating 

 levels of section. 



{cd), ten young, 

 dashes), ten animals 



