AGE DIFFERENCES IN METABOLISM OF PLANARIA. 393 



These researches indicate that the rate of respiratory exchange 

 is very high at some certain stage in the embryonic development, 

 this stage probably differing in different animals, and falls 

 subsequently. Later it probably rises with increased functional 

 activity. It certainly seems to me that the metabolic rate of 

 embryos, especially vertebrate embryos, cannot vaildly be 

 compared with that of post-embryonic stages, owing to the 

 enormous differences in functional activity which exist between 

 two such stages. The comparison of the metabolic rate of a 

 chick embryo with an adult hen seems to me a simple absurdity 

 when one reflects upon the difference in muscle tension alone at 

 the two stages. The fact that the total metabolic rate of chick 

 embryos was at no stage found to be less than that of the hen, 

 certainly indicates that if cells of the same degree of functional 

 activity could be compared in embryo and adult, the metabolic 

 rate of the embryonic cells would be vastly the greater. The 

 same criticism applies to cases where the young remain more or 

 less helpless after birth or hatching. 



In addition to these researches in which direct measurement of 

 the rate of oxygen consumption or carbon dioxide output of 

 young and old animals have been made, a considerable mass of 

 data is available in which another method was employed. This 

 is the direct susceptibility method extensively used in this 

 laboratory by Child and others; it consists in observing the 

 time of death of organisms in lethal solutions of various sub- 

 stances. We have brought forward a large amount of evidence 1 

 to indicate that the time of death in such solutions is an index 

 of metabolic rate, individuals of higher metabolic rate dying 

 first. When individuals of different ages are compared by this 

 method, it is invariably found that the time of death is shorter 

 the younger the individual, ahuays providing that the same degree 

 of functional activity is present in the animals which are being 



that the rate of respiratory exchange in the youngest embryos with which they 

 dealt very greatly exceeds that of the adult. It is only at certain later stages that 

 the rates of the two are approximately equal. The authors seem to have an idea 

 that the rate of respiratory exchange ought to be the same throughout develop- 

 ment, \vhereas their own and other data show that it is high in early stages and 

 declines as development proceeds. This decline is probably of the same nature 

 as that which organisms undergo from birth to maturity. 



1 A general resume of this evidence will be found in Child, '13. 



