422 NUTRITION OF FARM ANIMALS 



earlier results. As the technique of experimentation with iso- 

 lated nutrients has been developed by the work of Rohmann, 

 McCollum, Osborne and Mendel and others, it has become 

 evident that it is not the lipoids as such but some substance or 

 substances associated with them which are essential to con- 

 tinued growth. On the one hand, growth has been maintained, 

 for a considerable time at least, on a practically fat-free diet, 

 while on the other hand it has been shown that by no means 

 all fats are capable of exerting this favorable effect on growth. 



Both McCollum and Osborne and Mendel have found that rats fed 

 mixtures of purified nutrients containing no fat may grow normally 

 for a considerable time, but after about 75 or 100 days, and after 

 reaching perhaps f of the mature weight, there is a more or less abrupt 

 cessation of growth followed by a speedy decline in weight. Sub- 

 stantially the same result ensues when certain forms of fat (lard, beef 

 fat, olive oil, almond oil) are added to the ration, but if, on the other 

 hand, purified butter-fat, cod liver oil or certain other fats be added 

 to the ration of an animal which has ceased to grow and begun to 

 decline in weight this decline is promptly stopped and practically 

 normal growth resumed. These results indicate the existence of two 

 groups of fats, one of which aids growth while the other does not. 

 Evidently, therefore, the growth supporting property does, not reside 

 in the glycerids themselves but in some accompanying substances. 



499. Growth substances. On the basis of later investiga- 

 tions, McCollum 1 rejects Funk's hypothesis of the existence 

 of numerous specific " vitamins " and distinguished only two 

 growth substances (or classes of substances), both of which are 

 essential to growth. One, lipoid-soluble, which he calls fat- 

 soluble A, is associated with certain fats, while the other, called 

 water-soluble B, is soluble in water and apparently never asso- 

 ciated with fats. The fat-soluble A is absent from all vege- 

 table fats thus far examined. It is present in small but in- 

 sufficient amounts in the grains but appears to be relatively 

 abundant in the leaves of plants. 



That other factors than these specific growth substances may 

 markedly influence growth is, however, apparent from recent ex- 

 periments by Hart and McCollum 2 who found that the freedom 



1 Jour. Biol. Chem., 23 (1915), 181 and 231 ; 25 (1916), 105 ; Amer. Jour. Physiol., 

 41 (1916), 333 and 361. 



2 Jour. Biol. Chem., 19 (1914), 373. 



