THE VITAMINES IN THE ANIMAL KINGDOM 115 



not belong to the subject of vitamines) that for the correct feeding 

 of rats, it is necessary, above all, that the food should have the proper 

 composition. We know that a protein is necessary whose content 

 of amino acids is above reproach. We may say that casein, lactal- 

 bumin, 11 egg-albumin, muscle protein, as well as some plant proteins 

 have been shown to be adequate for this type of animal. Suzuki 

 and his co-workers (323) demonstrated that an artificial mixture of 

 amino acids does not permit of growth in the rat, while digested 

 meat (erepton) possesses this ability because of its vitamine content. 

 Aside from this, it may be connected with a more favorable relation- 

 ship of the amino acids to each other. In addition, we refer to the 

 excellent work of Osborne and Mendel, who have carefully studied 

 the value of various proteins on rats. 



Furthermore, it is necessary that the rats receive a sufficient and 

 properly composed salt mixture, and that the individual constituents 

 of the diet are present in proper proportion. This point, to be 

 mentioned again shortly, is of great significance. It cannot be said 

 with certainty that these basic conditions are always accurately 

 maintained; it is only in the work of the last six years that they have 

 been given their true value. All of the above mentioned factors must 

 be taken into account before undertaking a vitamine problem. 



We have already called attention to the work of Osborne and 

 Mendel in 1911-1912, in which they still clung to the idea that they 

 had solved the question of artificial nutrition. At first they did not 

 see that the addition of "protein-free milk" to the diet was an 

 unknown factor whose significance for nutrition could not be accu- 

 rately gauged at the time. This happened in spite of the work of 

 Stepp, who had already affirmed the presence of essential lipoids in 

 milk. Through the classical work of Hopkins and through our (324) 

 simultaneous demonstration of vitamine B in milk, it became clear 

 that something could adhere to the products made from milk; this 

 might explain the striking results obtained by Osborne and Mendel. 

 Hopkins showed that a small quantity of milk (a few cc.), 



11 Sure (322) has recently found that lactalbumin is not a complete protein 

 since it does not contain enough cystine and tyrosine. According to this, 

 it is obvious that the results of Emmett and Luros (I.e. 105), who suggested 

 the probability of a new vitamine, and who found that lactalbumin was 

 rendered complete when supplemented with protein-free-milk, could be 

 explained by the presence of cystine and tyrosine in the protein-free milk. 



