610 



METABOLISM 



are very marked variations in the units of which different proteins are 

 composed. If any one of these units should be essential for growth and 

 the organism be unable to manufacture the missing unit for itself, it 

 is clear that growth could not proceed however much protein not contain- 

 ing the necessary unit we might feed to the animal. It is an application 

 of the law of the minimum, and is analogous with the failure of growth 

 which has long been known to ensue when certain inorganic substances 

 are withheld from the growing animal. A diet might be perfectly bal- 

 anced as judged by comparison of the nitrogen intake and output, and 

 yet if it should fail to contain even one of the essential units and the 

 organism should be incapable of supplying this unit, then would the 

 diet be inadequate for growth. 



These important facts are the outcome of modern w r ork, and they 



Days 



Each division = 20 days. 



Fig. 183. Curves of growth of rats on basal rations plus the various proteins indicated. The 

 normal curve may be taken as that with casein (I). (Adapted from Lafayette B. Mendel and 

 T. B. Osborne.) 



have been established by observations on the growth of young animals 

 fed with a ''basal ration" to which were added mixtures of amino acids 

 or various proteins which differ considerably from one another in the 

 nature of the units entering into their make-up. In such experiments 

 the periods during which growth is observed must be prolonged, since 

 a transient increase in weight might depend merely on repair processes 

 occurring in tissues which had previously for some reason been brought 

 below par. 



Among the most important observations have been those of F. Gow- 

 land Hopkins, Lafayette B. Mendel and T. B. Osborne 8 and of McCollum 9 

 and his collaborators. The animals chosen for Mendel and Osborne 's 

 experiments were young white rats. Large batches of these animals 



