Take, for example, the single question of peptonization by gastric digestion. What is the nature of the process? Is the proteid transformed into a soluble and diffusible peptone as a result of hydration and cleavage, or is it a transformation which results from a simple depolymerization of the proteid molecule, _i.e._, are we to consider albumin and peptone as isomeric bodies? These questions, on which physiologists seem loath to agree, can certainly be answered definitely; not, however, by arguments but by careful experimentation, in which the composition of the proteid undergoing digestion must be a necessary preliminary factor, and the composition of the resultant product, or products, a secondary factor of equal importance. Further, the question needs to be answered not with reference to one proteid merely, but with reference to every proteid capable of digestion by either gastric or pancreatic juice. When these questions have been fully answered in this manner, we shall have positive data to deal with, and our conclusions will rest upon a foundation of fact not easily set aside. This is one of the problems upon which I have been at work for some years, and although progress may in one sense be slow, yet it is sure and gives results of no uncertain character. First, then, let us consider briefly the nature of the proteids whose proteolysis we may be interested in; remembering, however, that in so doing we can merely touch upon the points essential for our purpose. Allow me to say in parenthesis that there is being published in Moscow a work on proteids alone of five volumes, 900 pages each, which it is supposed will constitute an exhaustive treatise of the subject.[50] [50] Die Einheit der Proteinstoffe. Historische u. experimentelle Untersuchungen, by L. Morokhowetz. If we attempt to classify all of the proteid bodies hitherto discovered and studied we are at once confronted with a problem of no small proportions. So varied are they in their reactions, solubilities, and behavior toward general reagents, so inclined to merge into each other by almost insensible gradations that it becomes an extremely difficult matter to make an arrangement that will satisfy all the requirements of the case. I have to suggest, however, the following classification, which is merely a modification of several existing ones, based primarily upon chemical composition, and solubility in the more common menstruums. Proteids may first be divided into three main groups as follows: I. _Simple Proteids._--Composed of carbon, hydrogen, nitrogen, sulphur, and oxygen, and yielding by decomposition aromatic bodies such as tyrosin, phenol, indol, etc. II. _Compound Proteids._--Composed of a simple proteid united to some non-proteid body. III. _Albuminoids._--A class of nitrogenous bodies related to and derived from proteids, but differing especially from the latter by great resistance to the ordinary solvents of true proteids.