You will pardon me if I specially emphasize in this connection the fact that advance along the present lines was not rapid until physiologists began to appreciate the importance of investigating the chemico-physiological problems of digestion by accurate chemical methods. Something more than simple test-tube study, or even experimental work on animals, is required in dealing with the changes which complex proteids undergo in gastric and pancreatic digestion. The nature and chemical composition of the proteids undergoing digestion, as well as of the resultant products, are necessary preliminaries to any rightful interpretation of the changes accompanying digestive proteolysis; but physiology has been slow to appreciate the significance of this fact, and, until recently, has done very little to remedy the noticeable lack of accurate knowledge regarding the composition and nature of the proteid and albuminoid substances which play such an important part in the life-history of the human organism, either as food or as vital constituents of the physiologically active and inactive tissues. This is to be greatly deprecated, since our understanding of the nature of proteolysis, of the mode of action of the enzymes or ferments involved, and of the relationships of the products formed, is dependent mainly upon an accurate determination of the exact changes in chemical composition which accompany each step in the proteolytic process. How otherwise can we hope to attain a proper appreciation of the real points of difference between bodies so closely related as those composing the large group of proteids and albuminoids? Surely, in no other way can we measure the nature or extent of the changes involved in the various phases of proteolysis than by a thorough study of chemical composition and constitution, as well as of chemical reactions and general properties.