GROWTH 395 



distinctly less than would be expected from Rubner's results 

 on the utilization of pure nutrients by mature animals. In the 

 case of swine, moreover, the utilization appears to be even less 

 than that of the metabolizable energy of grain by mature ani- 

 mals, although the contrary would naturally have been antici- 

 pated. The results with older animals, while far from conclusive, 

 seem, if anything, to indicate a lower utilization by younger 

 animals as compared with older ones and at any rate fail to 

 show that it is any greater in the former case. 



The results on embryonic growth show a relatively large 

 expenditure of energy in development and indicate a compara- 

 tively low utilization of energy. This large expenditure of 

 energy in development seems to be required chiefly for the 

 organization, in the broader sense, of the embryonic structure 

 rather than for the mere chemical transformation of egg sub- 

 stances, and it seems to be relatively greater in the young as 

 compared with the more mature embryo. 



477. Provisional hypothesis. While it would be rash to 

 draw any final conclusions from the foregoing data, it may be 

 permissible to formulate a working hypothesis to the effect 

 that the conversion of feed protein (including the protein of 

 the egg) into tissue requires a considerably greater relative 

 expenditure of energy than does the conversion of surplus feed 

 into fat, the difference representing what might be called the 

 work of organization, i.e., the formation of organized structure 

 in the young animal and especially in the embryo. It has 

 been shown (463) that the rate of growth decreases rapidly 

 with increasing age. Accordingly, the work of organizing new 

 protein tissue, so far as this is measured by the storage of pro- 

 tein, must constitute a steadily diminishing proportion of the 

 total energy expenditure of the organism, since as the animal 

 grows older the increase consists to a diminishing extent of 

 protein and to an increasing extent of fat. The percentage 

 utilization of the feed energy would therefore, upon this hypothe- 

 sis, tend to increase. It would be least immediately after birth 

 and after two to four months would become relatively small, 

 corresponding to the changing character of the gain. Probably 

 by the time an animal has been weaned and is consuming the 

 normal feed of its species, the percentage utilization of the feed 

 energy might be assumed to be not much less than that ex- 



