94 



SENESCENCE AND REJUVENESCENCE 



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susceptibility of the newly hatched worms is much greater than 

 that of the full-grown animals, disintegration of the former being 



far advanced before it begins in 

 the latter. Since susceptibiUty meas- 

 ured by the higher concentrations of 

 the direct method varies with rate 

 of metaboUsm, the young animals 

 must have a much higher rate than 

 the old. 



But the method enables us to dis- 

 tinguish age differences in rate of 

 metabolism which are very much less 

 than these. In Fig. lo the curve ab 

 shows the susceptibility of ten worms 

 hatched within the twenty-four hours 

 preceding the beginning of the experi- 

 ment, and the curve cd the suscepti- 

 biHty of ten animals four days after 

 hatching and without food. Here 

 the difference in size between the 

 animals of the two lots is much 

 less than in the preceding case, the 

 younger worms being two millimeters, 

 the older three and one-half milU- 

 meters long. The figure shows that 

 the susceptibility of the newly 

 hatched animals is consider- 

 ably greater, i.e., their rate of metab- 

 olism is higher than that of the 

 animals four days after hatching. 

 Since the differences in susceptibility 

 as shown in Fig. lo are considerable 

 for four days' time, it is evident that the rate of metaboUsm must 

 decrease rapidly after hatching. 



The young worms are capable of movement before they emerge 

 from the egg capsules, and by opening the capsules with fine needles 

 it is possible to obtain young worms of various stages before hatch- 



FiGS. 7, 8. — Planaria maculata 

 and P. velata. 



