96 



SENESCENCE AND REJUVENESCENCE 



at this size and it is impossible to say whether the larger worms are 

 older in point of time than the smaller. They have, however, 

 attained a stage of growth and development which under anything 

 approaching natural conditions could be reached by the smaller 

 worms only after at least some weeks. 



The. larger, physiologically older worms begin to disintegrate 

 two hours later and also complete their disintegration one and one- 

 half hours later than the smaller ones. 

 In other words, their survival time is 

 about one-fifth greater than that of 

 the smaller worms. But in Fig. lo 

 above, the survival time of worms 

 four days after hatching is almost 

 one-half greater than that of worms 

 newly hatched, that is, the difference 

 in rate of metabolism between the 

 two lots of Fig. lo, which are only 

 four days apart, is much greater than 

 that between the two lots of Fig. ii, 

 which represent physiological condi- 

 tions several weeks apart in terms of 

 time. Clearly the rate of metabolism 

 decreases very much more slowly in 

 the larger, older worms than in the 

 stages immediately following hatch- 

 ing. A comparison of Figs. lo and 

 II also shows, as does Fig. 9, the 

 great difference in susceptibility be- 

 tween very young and full-grown 

 animals. 

 These results are in complete agreement with the observations 

 of Minot ('08) and others on the rate of growth in mammals and 

 birds. The rate of growth as measured by the percentage incre- 

 ment is highest in the youngest animals and decreases with advan- 

 cing age. As Minot says, " the period of youth is the period of most 

 rapid decline." And now we find this to be true, not only for the 

 rate of growth in the higher animals, but for the rate of metabolism 



Hours i li 2i 3j 



Fig. 10. — Susceptibility of Pla- 

 naria maculata to KCN o.ooi 

 mol.: ah, worms hatched within 

 24 hours; cd, worms four days 

 after hatching. 



