NUTRITION. 309 



foods, one of which was more suitable for inducing growth than the 

 other. Although the rats usually ate some of each food offered them, 

 only one failed to select enough of the better food to induce good 

 growth; and it is possible that if the experiment had been continued 

 a little longer this one animal would eventually have learned to make 

 a wiser choice in his diet. 



Small, yet measurable, quantities of nitrogenous products are added 

 to our "synthetic" diets when protein-free milk, yeast, corn germ, or 

 wheat embryo are used to furnish the vitamine. In studying the 

 nutritive value of isolated proteins, as well as that of proteins as they 

 occur in animal or vegetable products, this small amount of protein, 

 added with the indispensable vitamine, to a certain extent limits the 

 conclusions that can be drawn from such experiments; for it is never 

 absolutely certain that the good effect produced by the vitamine may 

 not be in part at least due to a favorable supplementing effect by the 

 protein in the vitamine-containing product. Accordingly we have 

 attempted to obtain a vitamine preparation freer from protein than 

 dried yeast, or wheat embryo, which we have usually used in the past 

 as sources of vitamine. This work is in progress, but we have not yet 

 succeeded in securing the desired product. 



In our report for 1917 we referred to the good growth of rats when 

 only 1.5 to 2 per cent of yeast supplied all of the water-soluble vitamine 

 of the diet. Twenty-five of these animals ultimately reached a very 

 large size, but all failed to breed when mated with rats raised on the 

 yeast diets, and only four proved fertile with rats raised on the ordinary 

 "mixed food" fed to our stock colony. Even changing the rats from 

 the yeast-vitamine diet to this stock diet failed to render them fertile. 

 Professor H. H. Donaldson kindly examined the reproductive organs 

 of these animals and reported that in the testes of the four animals 

 which he had then examined he found no spermatozoa, no tubular 

 tissue, and an excess of interstitial tissue. His report on the females 

 has not yet been received. This surprising result opens up a wide 

 field for the investigation of the relationship between the vitamine 

 content of the diet and the fertility of the animal. 



In our early feeding experiments several rats grew well on mixtures 

 of isolated foodstuffs which were supposedly vitamine-free. Our more 

 recent knowledge of the need of vitamines in the diet supports the con- 

 viction that these rats received vitamine from some unknown source. 

 It has been suggested by other investigators that the commercial lac- 

 tose in their diets contains not inconsiderable quantities of vitamine. 

 We have accordingly undertaken a study of this question, for its 

 answer may give a clue to the nature of the water-soluble vitamine. 



As long ago as 1913 we found that, in contrast with various other 

 fats, butter fat contains something absolutely essential for the con- 



