226 THE VITAMINS 



that "ether extraction of plant tissue does not remove the substance 

 essential for growth which is contained in butter fat. The obvious work- 

 ing hypothesis must for the present assume that the fat-soluble A is in 

 chemical union in the plant tissues, and in a complex which is not 

 soluble in fat or in ether. In digestion and absorption it is set free and 

 being readily soluble in fats, thereafter accompanies the fats in the 

 animal body." 



That neither vitamin A nor B could be synthesized by the animal 

 was indicated by the failure of rats on diets deficient in one or the other 

 of these vitamins successfully to suckle their young except through a 

 sacrifice of their own tissue and then to only a limited extent (McCol- 

 lum, Simmonds and Pitz, 1916c). 



The foregoing sketch attempts to review the most noteworthy 

 features of the literature of vitamin A in chronological sequence to the 

 end of 1917. Later investigations have in many cases proceeded along 

 such distinct lines that it seems best to consider them under separate 

 phases rather than to continue further the chronological development of 

 the whole subject. 



Physical and Chemical Properties of Vitamin A and Progress 

 toward its Isolation 



Solubility and Extraction. — As noted above, an apparent difference 

 between the solubility of vitamin A as it occurs in animal fats and in 

 plant tissues was early reported. The data, however, do not seem entirely 

 concordant. 



We have seen that the substance was discovered through its presence 

 in butter fat, egg-yolk fat, and cod-liver oil. In all these (and doubtless 

 many similar) cases, the vitamin A dissolves readily in ether along with 

 the accompanying fat. Osborne and Mendel (1919, 1919d) obtained 

 vegetable fats which appeared to be rich in vitamin A when they 

 extracted, with U.S. P. ether, spinach leaf or young clover, previously 

 dried in a current of air at 60° C. This ether extract was a dark green, 

 greasy or oily mixture evidently containing the fat of the plant tissue 

 and such other material as was soluble in the ether used as solvent, or in 

 the mixture of ether and fat which must have been present so long as 

 fat was still being extracted from the dried tissue by the ether. It is well 

 known, but not always remembered, that an ether extract may contain 

 substances which are not appreciably soluble in pure ether but are dis- 

 solved by the mixture of fat and ether which results when a plant or 

 animal tissue containing fat is subjected to ether extraction. This source 

 of possible confusion should be kept clearly in mind when dealing with 



