400 L. H. HYMAN. 



compared with older organisms. The fact further that the 

 susceptibility of young animals to a number of toxic substances 

 is greater than that of old could scarcely be supposed to be due 

 to a greater percentage of inert materials in the older individuals, 

 An interesting point bi ought out in these experiments is that 

 the difference between the asexual and the sexual young and 

 their respective adults is of about the same magnitude, when 

 considered the same length of time after feeding. Worms 

 produced by fission are therefore as truly "young" as those 



* 



which develop from the egg. 



In previous papers of this series (Hyman, '196, 'IQC), it was 

 shown that planarians which have been starved seven or eight 

 weeks and pieces of planaria which have undergone regeneration 

 have a much higher rate of oxygen consumption than ordinary 

 fed worms, all tests being made, of course, a few days after 

 feeding. Starved, regenerated and young worms therefore have 

 this physiological characteristic in common: their metabolic 

 rate is higher than that of large fed worms. That of starved 

 ones is highest, regenerated ones next, and young, w T hen produced 

 from the egg or simple fission, least. It therefore appears that 

 the metabolic rate of reduced forms depends primarily upon the 

 amount of reorganization involved in their production, and is 

 proportional to the degree of reorganization which has taken 

 place. As a further illustration of this may be cited the much 

 higher metabolic rate of the asexual young of Planaria velata 

 than those of P. dorotocephala, presumably because much more 

 extensive changes are involved in giving rise to the former. 

 The evidence presented in these papers clearly supports the 

 view which has been long maintained by Child that such 

 reorganizations due to whatever cause are rejuvenating trans- 

 formations, restoring the organism to a physiological condition 

 resembling that of the young. 



V. SUMMARY. 



i. The young of Planaria dorotocephala produced by simple 

 fission were found to consume 15 to 55 per cent, more oxygen 

 than large worms, the difference depending upon the length of 

 time w r hich had elapsed between the last feeding and the time 

 of testing. 



