AGE DIFFERENCES IN METABOLISM OF PLANARIA. 39! 



decreases more slowly. According to Benedict and Talbot ('14, 

 '15), the metabolism of infants is about the same as that of 

 adults, but the results were very variable. It seems highly 

 probable that the metabolism of infants cannot properly be 

 compared with that at other ages because: (i), the heat-regu- 

 lating mechanisms of infants are known to be very imperfect; 

 (2), infants commonly have relatively more fat than is present 

 in normal individuals of other ages; and (3), the muscle tone of 

 the voluntary muscles of infants must be lower than it is at 

 other ages. Since a large part of the heat production of mam- 

 mals originates in the voluntary muscles, this difference in 

 muscle tone alone makes impossible any real comparison of the 

 metabolism of infants with that of later stages of ontogeny when 

 the muscles are in full use. 



A few researches have been carried out on mammals other 

 than man. Thus Slowtzoff ('03) working on dogs found that 

 the oxygen consumption and carbon dioxide production per 

 kilogram per minute is greater in small than in large individuals. 

 A. V. and A. M. Hill ('13) determined the same relation in rats. 

 Among birds, Bohr and Hasselbalch ('oo) observed that the 

 carbon dioxide production per kilogram per hour is considerably 

 higher in newly hatched chicks than in the adult hen. 



These researches are sufficiently numerous to establish the 

 generalization that smaller (younger) individuals have a higher 

 metabolic rate than larger (older) ones. The investigations on 

 man clearly show that the difference is due to age and not to 

 size. Age is also probably the determining factor in the meta- 

 bolic difference found between large and small species, since, in 

 general, species which grow to a large size must be older by the 

 time they have attained that size than are adult individuals of 

 small species. 



How far back in the ontogeny can this generalization be 

 carried? At what point in the ontogeny does the metabolic 

 rate attain its highest value? Few researches have been carried 

 out upon these points. The eggs of animals are probably cells 

 of very low metabolic rate. After fertilization, the metabolic 

 rate gradually rises, as development proceeds, up to a certain 

 point and then falls. In the sea-urchin egg (Arbacia punctulata), 



