74 FEEDING EXPERIMENTS WITH ISOLATED EOOD-SUBSTANCES. 



identical conditions from the age of 38 days, except that rat 238 

 (see Chart LVI) was fed with a paste containing casein and pro- 

 tein-free milk, while in the food of 240 (see Chart CXIII) the casein 

 was replaced by gliadin. 



Plate 2 shows rat 220, K, fed on gliadin and protein-free milk 

 but weighing only 58 grams, although 148 days old, and, for con- 

 trast, rat 168, D, of approximately the size normal for the age of 

 rat 220, is also shown. Figure F shows a normally nourished rat of 

 the same weight as rat 220. This picture is introduced to show that 

 rat 220 has the appearance of a normal rat of corresponding size and 

 weight. All these pictures were taken on exactly the same scale and 

 afford a ready comparison of the relative sizes of the animals. 



The interesting photographs of underfed cattle published by 

 Waters, on the contrary, make the change of form in his under- 

 nourished animals of stationary weight quite apparent. We are, 

 however, not prepared to assert that careful measurements of our 

 stunted rats will not disclose some trace of similar changes in skeletal 

 form. They must be slight at most; for we have often compared 

 animals long maintained at small stature with properly grown animals 

 which have just reached the same weight, without detecting any devi- 

 ation from the youthful form in so far as one could judge by mere 

 visual inspection. The photographs speak in the same sense. 



The point on which we lay great stress in the foregoing experi- 

 ments is the fact that the stunting is not attributable primarily to 

 under-feeding. Our dwarfed rats have as a rule eaten as adequately 

 as normally nourished animals of the same size. The energy factor, as 

 such, thus drops out of the problem. In this respect the experiments 

 are not comparable with those of Waters and of Aron, both of whom 

 accomplished their results by underfeeding with adequate food mate- 

 rials. In our experiments the ' ' energy requirement for maintenance' ' 

 and the "energy requirement for growth," which together are essen- 

 tial to the developing organism, were both supplied. The rats did not 

 grow primarily at the expense of stored tissue materials : they failed 

 to grow in any sense. We are obviously dealing with some other feature 

 than insufficient energy supply. The numerous illustrative experi- 

 ments which will be cited later are accordingly to be interpreted as 

 instances of maintenance without growth. If it is true that growth 

 can only continue when the energy intake exceeds the mere main- 

 tenance requirement, it is equally true that an excess of calories does 

 not per sc insure growth in a suitable animal. Here then is the 

 opportunity to ascertain and differentiate some of the essential qual- 

 itative factors: protein, inorganic salts, etc. their minimum and 

 optimum values. 



