INTRODUCTION 3 



and a nitrogen-free part. The use and value of the former portion is 

 universally admitted, as nitrogen is an absolutely essential constituent 

 of all living tissue, but does the non-nitrogenous rest play a special 

 part in the metabolic processes which cannot be performed by non- 

 nitrogenous material arising from the breakdown of either carbo- 

 hydrates or fat ? Does this non-nitrogenous part of the protein molecule 

 simply serve as one of the sources of the energy supply, or does it, 

 owing perhaps to a more highly specific nature, play an intimate part 

 in the synthesis of fresh protein material ? 



