VITAMIN A 223 



suggested the idea that natural food materials contain other essential 

 ingredients than those hitherto recognized is shown by his statement 

 in 1906 noted in Chapter I. The delay in publishing the remarkable 

 results which he had obtained was explained in a note regarding his 

 early experiments in which Hopkins (1921) stated, "The experiments 

 which I described in 1912 followed upon a long experience of the 

 effects of adding tissue extracts and especially fractionated yeast prep- 

 arations to purified diets. Looking back to one's experience in these 

 years during which startling successes were mingled with puzzling fail- 

 ures — failures which led to delay in publication, I realize that the ab- 

 sence of all knowledge concerning the factor associated with fats was 

 the cause of any experimental contradictions. Had I possessed the scien- 

 tific vision which afterwards led McCollum and Osborne and Mendel 

 to recognize the existence of this, I should have reached full convic- 

 tion as to the reality of my results long before I did. In the synthetic 

 diets employed by me the proteins and the carbohydrates were purified 

 to the utmost, but I used little or no discrimination with regard to 

 the fats." 



The work of McCollum and of Osborne and Mendel to which Hop- 

 kins here refers, appeared almost simultaneously in 1913. McCollum 

 and Davis employed rations of purified casein, carbohydrates, and vari- 

 ous salt mixtures; and the same rations in which part of the carbo- 

 hydrate was replaced by lard. It was found that with certain proportions 

 of the various ingredients normal growth of the animals for periods 

 varying from 70 to 120 days often resulted, while beyond that time 

 little or no increase in body weight could be induced, although the 

 animals remained in apparently good nutritive condition for some time. 

 After numerous attempts to promote further growth by adjustments 

 between the various ingredients of the food mixture, it was found 

 that resumption of growth occurred quite promptly after the introduc- 

 tion into the diet of the ether extract of tgg or butter. Negative results 

 were obtained with lard and olive oil, thus showing that the suspension 

 of growth was not due merely to the absence of fat from the diet. The 

 conclusions drawn at that time, as taken from the authors' discussion, 

 are as follows: "Whether the resumption of growth is the result of 

 supplying in the ether extract of egg or of butter some indispensable 

 organic complex of the chemical nature of the lipins, or is the result 

 of a stimulating action of some substance accompanying the lipins can 

 not be decided from the data available. . . . Our observation that ether 

 extracts from certain sources improve the condition of animals on such 

 rations strongly supports the belief that there are certain accessory 



