98 



SENESCENCE AND REJUVENESCENCE 



Stages 



The worms which are so much "older" in point of time show 

 only a slightly greater resistance, i.e., a slightly lower rate of metab- 

 ohsm than the worms of the "younger" lot. As a matter of fact, 

 the worms of the curve cb had been considerably older physiologi- 

 cally at an earlier period than they were at the time when the 

 comparison was made and had been undergoing rejuvenescence 

 in consequence of reduction. We cannot measure the age of such 



organisms in terms of 

 time unless we know 

 that they have been 

 growing old without 

 interruption, and even 

 then the rate of senes- 

 cence may vary with 

 conditions. 



On the other hand, 

 size, or, more strictly, 

 length — for in the later 

 stages the growth is 

 largely a growth in 

 length — is under the 

 usual conditions and 

 within certain limits, a 

 fairly good criterion of 

 physiological age. 

 Barring individual size 

 differences, which are 

 sHght, the length of the 

 animal is an index of 



Hours 1234 



Fig. 12. — Susceptibility of Planaria maculata 

 to KCN o.ooi mol.: ab, worms 9 mm. in length 

 and ten weeks after hatching; cb, worms 10 mm. in 

 length and at least one year after hatching. 



the amount of growth and development which has occurred, and 

 we find in general, as the preceding figures show, that the longer 

 animal has a lower rate of metabolism than the shorter. But it 

 does not follow that individuals of the same length always possess 

 the same rate of metabolism. A given size may be attained either 

 by growth from a smaller or reduction from a larger size, and the 

 physiological condition of the animal is not the same in the two 

 cases. But in a single stock, where all individuals have been under 



