The proteid molecule is presumably built up of amino-acids variously joined together, this synthesis being accomplished, doubtless, by the condensation of different types of amino-acids, in which the first of the above groups represents the more common method of union. We may indeed conjecture that such methods of condensation take place in the human body, in the epithelial cells of the intestine, and in the tissues in general; and that by such methods, construction of proteid is accomplished out of the various fragments split off by digestion, etc. In a tentative way, the principle may be illustrated by the fusion of leucin and glutaminic acid,--following Hofmeister’s suggestion,--in which a still larger complex is formed: : : --CO-:-NH--CH--CO--NH--CH--CO-:-NH-- : | | : C_{4}H_{9} (CH_{2})_{2} | CO·OH Leucin Glutaminic acid In this way, step by step, the proteid molecule is built up, and naturally in katabolism the proteid breaks down along certain definite lines of cleavage, with formation of katabolic products containing those groups, or chemical nuclei, which characterize the different proteid molecules. For it is to be clearly understood that there are many different forms of proteid, perhaps superficially alike, but possessed of physiological individuality. This is well illustrated by the two primary proteoses formed in digestion. As will be recalled, there are at first two proteoses produced, protoproteose and heteroproteose. These are, superficially at least, not radically unlike; they possess essentially the same percentage composition, but when broken down by vigorous chemical methods they show a totally different make-up. In other words, at the very beginning of digestion there is a splitting up of the proteid into two parts, which have quite a different chemical structure, as is clearly indicated by the difference in the character and amount of the decomposition products yielded by hydrolytic cleavage. Thus, heteroalbumose as derived from blood-fibrin contains 39 per cent of its total nitrogen in basic form, _i. e._, in a form which goes over into the basic bodies, arginin, lysin, and histidin, etc. On the other hand, protoalbumose from the same source yields hardly 25 per cent of basic nitrogen. Further, heteroalbumose yields only a very small amount of tyrosin, while protoalbumose gives on decomposition a large amount of this substance. Again, heteroalbumose furnishes a large yield of leucin and glycocoll, while protoalbumose gives no glycocoll and only a little leucin. Obviously, these two proteoses have an inner structure quite divergent one from the other, and owing to this fact they must play a quite different rôle in metabolism.