GROWTH 413 



quirements for growth is fragmentary and more or less con- 

 flicting. On the one side are investigations like Fingerling's 

 on cattle and Just's on sheep (486, 487) which appear to show 

 that it is possible for the animal to support what seems a normal 

 rate of growth upon a supply of protein little greater than the 

 maintenance requirement plus the amount actually stored. 

 On the other side stand the results of experiments and observa- 

 tions upon the fattening of immature animals, in which rations 

 at least approximately equal as to their content of net energy, 

 and therefore presumably equally effective for simple fattening, 

 have produced a greater increase in weight when they contained 

 relatively much more protein than the results of the other class 

 of experiments would indicate to be necessary. Further con- 

 sideration of this apparent conflict of evidence, however, shows 

 that the two classes of experiments are hardly comparable. 



For one thing, the experiments in which a relatively high 

 protein supply seemed advantageous were all fattening ex- 

 periments. The effect of the feed was measured by the gain in 

 live weight, which itself is a somewhat uncertain criterion, 

 while a considerable share of this increase was due to a storage 

 of fat rather than of protein. Fingerling's and Just's experi- 

 ments, on the contrary, relate distinctly to growth and the 

 comparisons are based on the actual amounts of protein tissue 

 produced, although it must be admitted that any experimental 

 errors would probably tend to make the excretion of nitrogen 

 appear too low, and therefore the gain of protein too high. 



Another important difference between the two classes of 

 experiments lies in the nature of the rations. In the metab- 

 olism experiments they were composed largely of commercially 

 pure nutrients such as starch, oil, etc., with only the amount 

 of roughage necessary to supply bulk, and in particular, the 

 variations in the protein supply were effected by changes in 

 the amount of commercially pure wheat gluten. In the fatten- 

 ing experiments, on the contrary, the higher protein content of 

 the rations was obtained by the use of the ordinary protein- 

 rich feeding stuffs. What these experiments really show is that 

 a larger proportion of these feeding stuffs was advantageous, 

 but it does not necessarily follow that this advantage was due 

 to the added protein. For one thing such a modification of 

 the rations must have affected the ash supply to a certain extent. 



