NORMAL PROCESSES OF ENERGY METABOLISM 619 



ing the development of any of these insects, but a portion of the energy 

 (according to Farkas approximately one-third) arises from the oxidation 

 of proteins to uric acid. Both Tichomiroff for the silkworm egg and 

 Weinland for the blow-fly recorded a reduction of the glycogen content 

 of the egg, but Weinland believes this may have been converted to chitin. 

 There is no evidence, he says, that glycogen has served as a source of 

 energy. 



Our information as to what material is the source of energy for the 

 mammalian embryo is extremely scanty. Cohnstein and Zuntz analyzed 

 the blood in the umbilical artery and vein of the sheep embryo for oxygen 

 and carbon dioxid, and noted a difference of 4.67 vols. per cent O 2 and 

 4.72 vols. per cent CO 2 in one case and 4.0 vols. per cent 2 and 6.5 

 vols. per cent CO 2 in another. These figures would give respiratory quo- 

 tients of 1.01 and 1.6 respectively for the two embryos. It is doubtful 

 whether these figures are to be trusted, since on the basis of the same 

 analyses the authors claim a metabolism for the embryo of only one-fourth 

 to one-sixth as much per unit of weight as for the mother. The quotients 

 agree, however, with those found by Bohr on the embryo of the guinea 

 pig. Bohr took the difference between the total gaseous exchange of the 

 pregnant animal (after operation under anesthesia and immersed in a 

 warm bath) before and after clamping off a single umbilicus. The res- 

 piratory quotient indicated for the embryo was always in the neighbor- 

 hood of unity. Oddi and Vicarelli report also a progressive increase in 

 the course of pregnancy in the mouse. According to these observations, 

 therefore, the most diffusible of the foodstuffs, the one most readily passed 

 through the placenta is probably the source of energy for the mammalian 

 embryo. There is no satisfactory evidence as yet that proteins participate 

 to any considerable extent in furnishing such energy. 



3. Metabolism of Post-embryonic Growth. While metabolism is cer- 

 tainly more active in the youthful organism than in the adult it is by no 

 means proved that the growth per se calls for any expenditure of energy. 

 In recent times the view seems in fact to have gained rather general ac- 

 ceptance that the large metabolism of the young is necessary in the 

 interest of heat regulation. At the same time the propensity to grow, 

 which is the certain sign of youth in health, may be given a sort of 

 energy index. There is a considerable body of evidence that growth in 

 a given genus is proportional to the potential energy of the food consumed, 

 and the proportion of gain in weight to energy intake may be quite similar 

 in different genera. 8 It would seem that the growth impulse which, in 

 some way not at all understood, directs and governs developmental events 

 through the processes of nutrition, is geared, so to speak, at a very similar 



8 This statement, in view of recent developments in the realm of the chemically 

 unknown accessory substances ( vitamines), must be guarded by the saving proviso that 

 an adequacy of these several substances is assumed. 



