230 



Comparative Animal Physiology 



off of oxygen consumption, thereby giving some indication as to the nature of 

 the mechanism: whether the decrease is due to the utiHzation of food material, 

 or to accumulation of deleterious metabolites, crowding, etc. 



Respiratory exchange of embryonic mammals was regarded by Bohr and 

 Hasselbalch^^'- to be directly proportional to weight rather than surface area. 

 Barcroft's data^' on sheep and goat fetuses, obtained by measuring oxygen 

 tensions in the umbilical artery and veins, indicate furthermore that the oxygen 

 uptake is lower (0.0026 cc./gm./min.) than would be expected when com- 

 pared with adult tissues, and may be considered a reflection of the adequate 

 temperature conditions provided by the parent, thereby relieving the offspring 

 of the necessity of using energy for warmth alone. The shift in dissociation 



100 200 }00 lOOO 2000 5000 



Fig. 48. Double log plot of average total heat production and average 

 body weight. From Benedict.^' 



curves to permit the fetus to pick up, and the parent to give up, oxygen more 

 readily permits adequate gas exchange at these metabolic levels^^ (see Chapter 

 9). Bohr^' ingeniously controlled the umbilical blood flow in urethanized 

 guinea pigs, at the same time taking tracheal samples, and demonstrated that 

 the gas exchange of the parent and that of the fetus are almost the same. 

 Embryos of snakes^- and turtles-" undergo a gradual decrease in oxygen 

 consumption with development; in turtle embryos there is a possibility that a 

 considerable part of the oxygen consumption is associated with the catabolism 

 of quantities of protein. One would expect a new metabolic peak to develop 

 after the young are hatched. 



