SUBSTRATES OF GROWTH 



491 



balanced by tissue-repair, and continuous loss of tissue on an otherwise 

 abundant diet is the inevitable outcome. 



FIG. 33. A and B show the contrast between two rats of the same age, one of 

 which, B, has been stunted by receiving a diet (protein-free milk and gliadin) , deficient 

 in lysine. The lower two pictures afford a comparison between two rats of the same 

 weight but widely differing in age. The older, stunted rat, B, has not lost the character- 

 istic proportions of the younger animal, C. (After Under hill.) 



It has long been realized that Gelatin is not in itself an adequate 

 protein for the maintenance of nitrogenous equilibrium, although it is 

 a " sparer of protein/' i. e., can furnish a portion, but not the whole of 

 the nitrogen in the diet. We now recognize that this is due to the 

 absence of tyrosine and tryptophane from the molecule of this protein. 



