194 THE FEEDING OF ANIMALS 



animal, growth was not secured. When, however, what 

 was termed "protein free" milk was used in connection 

 with such a preparation, normal growth resulted. This 

 result, observed by Mendel and Osborn, led up to a 

 series of investigations in which Hart and McCollum have 

 taken a prominent part. It now appears from abundant 

 data that two classes of growth-promoting substances 

 exist, which have been termed Fat-soluble A and Water- 

 soluble B, terms which are temporary until these bodies 

 have been identified. The proof of the existence of these 

 bodies has been illustrated as follows (McCollum): If to 

 a mixture of purified proteins, carbohydrates, and salt 

 mixtures containing all the salts found in the animal 

 body there is added either a small amount of egg yolk 

 or milk powder, growth proceeds normally; whereas the 

 mixture of nutrients before such addition fails to produce 

 growth. If the dried egg yolk is extracted with ether to 

 remove the fat, the addition of the residue does not give the 

 desired result. The addition of the fat alone also is shown 

 to be futile. If, however, there is added to the nutritive 

 mixture along with the extracted egg-fat a water-extract 

 of the fat-free yolk residue, growth is normal. Similar 

 results occur with other substances, such as milk powder. 

 This is the basis for the classification into Fat-soluble A 

 and Water-soluble B, both of which are essential to 

 growth. It appears that the milk of an animal which has 

 been fed on purified nutrients fails to sustain her young, 

 showing that these growth-promoting substances are 

 transferred from the food to the mother's milk and are not 

 synthesized within the body. 



It seems certain that the disease known as beri-beri, 

 brought about by a restricted diet of polished rice, is due 

 to the absence of one of these classes, and it is probable 



