302 CARNEGIE INSTITUTION OF WASHINGTON. 



died. The remaining animals, which are now about 300 days old, 

 are still in good physical condition, although many of them weigh 

 little more than one-third as much as do normally fed animals of the 

 same age. The plan followed was to first feed the rat with a food 

 incapable of promoting growth until it declined about 10 grams in 

 weight and then to give it our "milk-powder food" for a day or two 

 until it had regained its lost weight. This procedure has made it 

 necessary to weigh the rats daily, so that the foods could be changed 

 at the proper time. 



Two female rats, which had failed to grow above 170 grams during 

 the progress of experiments carried on in the previous year, were kept 

 at constant weight by the above-described method, until one was 480 

 and the other 554 days old. On a normal mixed diet the younger one 

 has subsequently gained 37 per cent, reaching a weight of 220 grams — 

 somewhat more than the average maximum of female rats which have 

 grown normally on mixed diets. The older rat has, at the time of 

 writing, gained 18 per cent, and is still gaining, although at the time 

 this deferred growth began it had reached about twice the age at 

 which rats normally cease to grow. 



These experiments indicate that animals which have failed to grow 

 in consequence of defective diets do not early lose the capacity to 

 grow to their full normal size. We shall soon be able to control these 

 observations by testing other animals which have been permitted to 

 make much less growth than the two just described. 



In our last report we stated that when rats are fed on our ''protein- 

 free milk" diets, containing an adequate protein, they grow normally 

 until they reach about three-quarters of their natural size. After 

 this they soon decline and die, unless supplied with the "milk food" 

 or with a normal mixed diet. This failure is a direct result of growth, 

 for rats fed on the "protein-free milk" foods which contain an inade- 

 quate protein, such as gliadin, live very much longer. We thus have 

 the curious fact that rats which grow die, while those that do not 

 grow live. A very large number of experiments have been carried 

 on during the past year for the purpose of discovering the cause of 

 the failure of the rats to complete their normal growth on the "pro- 

 tein-free milk" foods. Control experiments with many rats fed on 

 the "milk food" have shown that this contains everything essential 

 for growth. Not only do such rats grow rapidly to full size, but they 

 produce large litters of young, which in turn grow normally and are 

 fertile when fed exclusively on the same "milk food." It is thus 

 evident that the milk food contains something essential for perfect 

 growth which is lacking in the "protein-free milk" food. 



As it seemed fair to presume that the "protein-free milk" contains 

 all of the constituents of the milk except the protein, fat, and sus- 

 pended sohd matters (cell debris, bacteria, etc. ?), we attempted to 



