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SCIENCE 



[N. S. Vol. XXXIV. No. 882 



refer to our detailed publication (Part I.). 

 How successful this has been is best answered 

 by the statement that rats have been main- 

 tained for many months at all ages with ap- 

 parent success. Far more important than the 

 ability to withstand confinement in a re- 

 stricted space has been the demonstration of 

 the possibility of maintaining rats on an 

 " artificial " food paste of unaltered uniform 

 composition during a large span of their life. 

 Herein we have apparently been far more suc- 

 cessful than any of our predecessors; for the 

 supposed monotony of diet has been the 

 stumbling block leading to failure in the 

 records of various investigators.^ Their ani- 

 mals have failed to eat and have declined as 

 an obvious result of insufficient food intake. 

 We are inclined to lend emphasis to the result 

 of the excellent hygienic environment and care 

 of our animals. And whereas nutritive de- 

 cline has commonly been attributed to the 

 anorexia consequent upon the monotony of 

 diet, we are more than ever inclined to shift 

 the explanation in many such cases to mal- 

 nutrition as a primary cause. From this point 

 of view improper diet and malnutrition may 

 be the occasion rather than the outcome of the 

 failure to eat — a distinction perhaps not sufii- 

 ciently recognized heretofore. 



As the criterion of the nutritive status of 

 the rats their body weight has been adopted, 

 and this has proved to be an advantageous 

 index. It soon became apparent that one must 

 distinguish sharply between maintenance and 

 growth in any such study of nutrition. The 

 white rat shows a very characteristic curve of 

 growth (plotted from the body weight) which 

 becomes practically stationary within 800 

 days. According to Donaldson the body weight 

 changes from 5 grams at birth to 270 grams 

 in the case of the male, or 225 grams in the 

 female at the age of 300 days. To judge of 

 the effect of a dietary regime by noting the 

 subsequent duration of life, as is still fre- 

 quently done by investigators, is misleading; 

 for the incidence of death may depend on the 

 ' These earlier studies are reviewed in Publica- 

 tion 156, Part I., Carnegie Institution of "Wash- 

 ington, 1911. 



previous nutritive condition — the store of fat 

 and glycogen — where food is insufficient. An 

 error less readily appreciated consists in de- 

 scribing the nutritive status as necessarily 

 satisfactory because an animal maintains an 

 undiminished body weight over long periods 

 under the conditions imposed. A man who 

 maintains his weight may be in excellent nu- 

 tritive condition; but a child which does like- 

 wise is failing to grow. Childhood demands 

 of a perfect ration the possibility of normal 

 growth, not simply maintenance. This can 

 not be emphasized too strongly. Furthermore, 

 growth in the sense of an increase in the size 

 of some structural part of the body or some 

 organ may proceed independently of the cor- 

 related development of the body as a whole. 

 Even with the existence of unquestionable 

 malnutrition, skeletal growth may manifest 

 itself in a conspicuous degree; so that the 

 length or height of an individual may mark- 

 edly increase while the total body weight re- 

 mains stationary or even declines. One part 

 of an organism may thrive at the expense of 

 other tissues. The complexity of these rela- 

 tionships of absolute and relative (or propor- 

 tionate) growth have likewise commanded at- 

 tention in our experiments. 



A study of physiological literature will 

 make it evident that no convincing reply has 

 been given to the question: can life be main- 

 tained and is growth possible with a single 

 protein in the dietary. " Protein " has been 

 used in this connection in a generic sense; 

 and one of the (chemically) simplest foods, 

 milk, contains at least two proteins of marked 

 individuality. Casein and lactalbumin are 

 chemically unlike. How widely two exten- 

 sively used food proteins may differ in their 

 chemical make-up is indicated below. 



The individuality of proteins of different 

 biological origin is further indicated by their 

 specific immunity reactions. The published 

 feeding experiments in which a single purified 

 protein has been administered to animals are 

 all limited in their duration to periods of days 

 or weeks which are too brief to furnish con- 

 vincing data. Indeed, one will scan the lit- 

 erature in vain for properly controlled experi- 



