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Biochemistry has continued to expand more and more rapidly each year, in stature as well as in scope. The subject-matter of biological chemistry, like that of biology itself, can be roughly divided into two parts; the static, or morphological, and the dynamic, or physiological. In some fields, notably in that of protein chemistry, the interdependence and collaboration between the organic and the biochemist are so intimate that it is impossible to say which is the organic chemist and which the biochemist. For the organic chemist the main focus of attention is the structure and configuration of these materials while, for the biochemist, the main problems are those of the behavior and function of these substances in organized, biological systems. Elementary Biochemistry is taught in universities. The first and older of these, Chemical Physiology, forms part of the course in mammalian Physiology, and caters primarily for the needs of medical and veterinary students. In the second course, much more recently introduced Biochemistry is taught as an independent scientific discipline, and without that emphasis on clinical problems with which it has usually been associated in the past, and which properly finds a place in Chemical Physiology. For students taking this second course there exists no suitable textbook, and it is primarily with their needs in mind that the present book has been written. With the needs of such students as these in mind I have invluded a short bibliography of review articles and books, mostly of recent date and written by experts in their respective fields. In Biochemistry, as in any young but rapidly expanding branch of science, there are fields in which facts are scanty, evidence contradictory and speculation rife. I cannot wholly subscribe to the doctrine that speculation is out of place in an elementary textbook, for there are many gaps in the subject, and unless these can in some way be bridged it is difficult or impossible to give a coherent account. Speculation plays and has always played an important part in the advancement of scientific knowledge, for no research worker gropes blindly after he knows not what; he invariably begins with certain reasonable possibilities in mind. In short, he speculates. To speculate unreasonably is worse than not speculate at all, but providing certain tests of reasonableness and compatibility are applied beforehand, speculation is a valuable tool, and one which finds a place in every scientific workshop..