240 E. V. McCOLLUM 



sugar glucose in abundance, for this is the sugar which the cells make 

 direct use of as a source of energy. Furthermore, it must contain suffi- 

 cient amounts of at least three, and in the case of some species, probably 

 four substances, the chemical natures of which we know nothing. We 

 recognize these dietary factors solely by observing the pathological changes 

 which result from the absence of one or another of them from the diet. 

 These substances have been most frequently designated, "vitamines," 

 following the suggestion of Funk. Since it is necessary to specify which 

 one is meant in any discussion of a specific nature, it has become the 

 practice to speak of them in algebraic terms as fat-soluble A, water- 

 soluble B and water-soluble C, which designate the substance which pro- 

 tects specifically against a type of ophthalmia, beri-beri and scurvy re- 

 spectively. The fourth substance in this class, whose existence was sug- 

 gested above, has not been conclusively shown to be distinct from the 

 first, or fat-soluble A, although there is much evidence that it is. It is 

 the organic factor which is concerned with the development and function- 

 ing of the osteoblast, and is therefore intimately concerned with the 

 etiology of rickets. 



The cells of living tissue have a remarkable power to synthesize many 

 complex organic compounds from relatively simple ones. Notable ex- 

 amples of this power are seen in the capacity of the body to synthesize 

 lecithin, cephalin and nucleic acid during growth, and the phosphorized 

 protein, casein, during milk formation, deriving for all these the neces- 

 sary phosphorus from the salts of orthophosphoric acid. All of these 

 are compounds too complex for synthesis by chemical procedure in the 

 laboratory. Although the tissues are remarkably efficient in the synthesis 

 of certain very complex organic compounds, they are powerless to affect 

 many others, even of a simpler nature, for their own preservation. Among 

 those which cannot originate by synthesis through the agency of the living 

 cells are the unidentified dietary factors which are concerned with the 

 etiology of the so-called "deficiency" diseases. This last statement needs 

 qualification to a certain extent. It has been shown that the rat cannot 

 be made to develop scurvy, even when the diet is entirely free from the 

 antiscorbutic substance, and that its liver under these conditions is rich 

 in this dietary factor. This species, therefore, is able to synthesize a 

 single one of the dietary factors which are concerned with the etiology 

 of the "deficiency" diseases. This is the only one which has been shown 

 to be capable of originating de novo within the body, and only the rat 

 and the western prairie dog of the United States have been demonstrated 

 to possess this power. It is certain that man, monkey, and guinea pig, 

 and apparently swine, cannot protect themselves against the development 

 of scurvy, when the antiscorbutic substance is lacking in their food. 



Under normal conditions growth follows definite laws, characteristic 

 of each species, which determine the rate at which growth will proceed 



