394 



NUTRITION OF FARM ANIMALS 



At the end of incubation the embryo * was separated from the yolk 

 sack and its contents and the energy of each determined with the fol- 

 lowing results : 



TABLE 90. UTILIZATION OF ENERGY IN INCUBATION 



In other words, 35 to 40 per cent of the energy of the egg substance 

 used was not recovered but escaped as heat. Comparison with the 

 loss of dry matter showed that the material thus katabolized consisted 

 substantially of fat. No loss of nitrogen was observed. 



Experiments on mammalian and reptilian embryos, especially 

 by Bohr, 2 by Murlin 3 and by Carpenter and Murlin, 4 appear in 

 accord with the foregoing conclusion, since they show that the 

 metabolism of the embryo per unit of weight is as great or 

 greater than that of the mature animal, despite the fact that the 

 maintenance requirement of the former must be decidedly less. 



The growth of the embryo consists essentially of the organ- 

 ization of protein tissue. The fact that there is no loss of nitro- 

 gen during incubation would indicate that chemically the 

 process is effected by a cleavage and resynthesis of protein 

 which appears to be a nearly iso thermic process (233, 367 d). 

 Apparently the organization of the protein into structure is 

 what calls for the large expenditure of energy. 



476. Summary. The experimental results mentioned in the 

 foregoing paragraphs may be briefly summarized in the follow- 

 ing statements : 



In the case of suckling animals, while no direct comparisons 

 of the same animal at different ages are available, the utilization 

 of the metabolizable energy of milk for growth appears to be 



1 Including the egg membranes. 



2 Skand. Arch. Physiol., 10 (1900), 413 ; 15 (1904), 23. 



3 Amer. Jour. Physiol. , 26 (1910), 134. 4 Arch. Inter. Med., 7 (1911), 184. 



