Respiration and Metaholism 



229 



oxygen consumption in a number of domestic animals has been demonstrated 

 by Brody and his co-workers; cattle, for example, reach their maximum metab- 

 olic rate between weaning and puberty, swine at the time of puberty, but 

 horses do not reach it until maturity.^i Comparable studies on human metab- 

 olism are summarized in the work of Krogh,-^-* indicating the maximum is 

 reached at two years of age; more recent data, however, would put this some- 

 what earlier, near the time of weaning.'^"' '"" This increasing metabolic rate 

 in young children is perhaps due to marked development of the musculature 

 during this period rather than to a general development of the body as a whole. 

 Breaks in the growth curves have been demonstrated in lower vertebrates as 

 well, particularly among Amphibia. The break in the growth curve for Rana 

 corresponds to the end of the gastrulation period.'^ The break in the otherwise 



1500 



1000 



500 



I 10" 



BODY NITROGEN (gm.) 



Fig. 47. Comparison of metabolic rate of various animals as a function of 

 body nitrogen. After Zeuthen.^ 



smooth exponential growth curve of Amhlystoma is rather prominent and 

 comes at the sixth or seventh day of development (Fig. 51).'^** Amerling's 

 data,^ based on the resistance to oxygen lack ("Zeitdauer"), support the con- 

 cept of increasing dependence on oxygen as the resistance decreases steadily 

 up to about the 12 mm. stage and then levels off more gradually. 



A correlation in rate of oxygen consumption with age may be applied to 

 cultures of microscopic organisms. Cultures of the protozoan Chilomonas 

 Paramecium decrease in oxygen consumption from 0.35 to 0.17 and O.ll cu. 

 mm. per hour per 10,000 individuals in cultures which age 24, 48, and 72 

 hours respectively.^^" It would be of interest to know whether subsequent 

 determinations on these cultures might show a further decrease or a leveling 



