Respiration and Metaholism 235 



in a comparison of newborn and adult dogs the oxygen consumption both of 

 brain tissue and of the intact animal was demonstrated to be significantly lower 

 in the pup."'^ Isolated fibroblasts of the chick heart survive anaerobically and 

 take up little oxygen during the first five days of development, but tissues from 

 older chicks become sensitive to oxygen lack"'^ and increase their rate of oxygen 

 consumption. 



In summary, we find that developing organisms tend to show a decrease in 

 oxygen consumption with increasing size, but in very early and embryonic 

 stages reflect profound developmental transitions involving structural changes, 

 tissue formation, and shifts in the types of respiratory mechanisms. A high 

 rate of oxygen consumption at fertilization and cleavage is usually followed 

 by a gradual decrease with embryonic development until birth or hatching, 

 after which the rate ascends to a maximum, to be succeeded by another slump 

 with advancing age. 



Oxygen Consumption in Relation to Species. The differences in absolute 

 consumption values between species have in a few instances been shown for 

 "races" within the species, a condition probably representing genetic differ- 

 entiation within the group. Certainly more such data are needed before any 

 general interpretations regarding their significance may be drawn. Metabolic 

 differences have been claimed for human populations,-'' but a re-evaluation 

 of the importance of the environmental factors as serious sources of disparity 

 should be made. In comparison with the standard basal metabolism values 

 (B.M.R.) for Caucasian men and women (United States), those of Mayan 

 Indian males are claimed to be 8 per cent higher, those of Miao (West China) 

 males 16 per cent higher, and those of Araucanian Mapuche (Chile) men 

 and women are claimed to be significantly higher than those of comparable 

 age groups in the United States. The metabolism values for Indian women 

 from the southern provinces of Tamil and Malaydis are consistently lower than 

 the figures for Caucasians. -''' 



Curiously, adult Drosophila melanogaster of the vestigial strain have been 

 shown to have a slightly higher oxygen uptake than normal wild type flies, 

 and the latter a higher consumption per weight than hybrids.-"^ Dobzhansky 

 and Poulson'*^ showed, in what was then considered Race A and Race B of 

 D. pseudoohscura (now regarded as D. pseudoohscura and D. persimilisX a 

 difference in gas exchange when measured at 25° and 14° C, their respective 

 environments. Although the oxygen uptake in the two races was the same at 

 14° C, Race A which normally inhabits the warmer climate had a lower 

 oxygen consumption at 25° C. than did Race B, a form of adaptation to the 

 higher temperature. Specific differences in minimum oxygen requirements 

 for several species of Drosophila have been reported, D. ohscura requiring 2.8 

 per cent oxygen in air, compared with D. melanogaster, which was claimed 

 to need only 1.6 per cent.*^' Recently the "killer" strain of Paramecium aurelia 

 was shown to have a higher oxygen uptake than the genetically different 

 "sensitive" strain.'^-" Owing to the profound if complex genetic control of 

 metabolic activity, it is to be expected that hereditarily distinct varieties of 

 organisms would show different and characteristic physiological behavior 

 patterns, particularly under conditions of metabolic stress, reflecting climatic 

 and other adaptations. 



Oxygen Consumption in Relation to Sex. Although sex differences in 



