620 



METABOLISM 



of the existence of this factor we owe to Stepp who found that mice 

 could not live for long on animal foods from which all fatty substances 

 had been thoroughly extracted by alcohol and ether. If the extract was 

 restored to the extracted food, this again became adequate. Hopkins 

 then showed in carefully controlled work that animals (rats) not only 

 failed to grow, but declined and died when they were fed on artificial 

 diet composed of the purified constituents of milk, although they might 

 eat voraciously. The addition of a few drops of milk, insufficient to 

 raise the energy or protein value appreciably, invariably caused normal 



... -_ *B" (water-soluble) factor. Note that growth ceases 



** 1S withdrawn. III. The time during which animals survive after removal of the 



A tactor at different ages, the uppermost curve being that obtained for rats that were nearly 

 mature before the factor was removed from the diet and the lower ones for less mature animals. 

 it u i C( ?. ntmuous c r ve is for young fed by mothers receiving "A" factor int their diets; 

 the broken-line curve is for young fed by mothers receiving none of this factor. (From Hopkins 

 and Chick.) 



growth to return. Osborne and Mendel also found that although the 

 rats in their experiments already referred to on page 609 grew for about 

 two months upon a diet containing protein, protein-free milk, starch and 

 lard, they ultimately declined, but that this could be avoided by substi- 

 tuting butter for the lard, and that the active substance was concen- 

 trated in the butter-fat portion of the butter. Later work by various 

 investigators showed that most animal fats, but not those of plants, 

 contain this essential, and it was hence called fat-soluble A factor or 



