Investigation has also shown that there is a considerable group of interactivities which are aroused in the body for purposes of offence and defence by the provoking causes of disease and by various poisons. This involves the production of numerous antibodies, etc., and this production is undoubtedly due to the chemical activity of various cells. The antibodies are then in the strictest sense of the term, internal secretions. But to confine ourselves to the chemical regulators of normal metabolism, in other words to the" hormones", our first inquiry should be as to the nature of these substances. They are specific chemical compounds produced by the activity of definite organs. They are for the most part of small molecular size and very stable. They produce their specific effect in infinitesimallysmall quantities, and they cannot act as antigens, which is to say, they do not cause the production of antibodies if they be injected into individuals other than those from whom they have been derived. True hormones have been classified according to the manner in which they act. Thus we have the formative and protective, growth-promoting and growth-retarding, stimulatory and inhibitory types. The term" ductless gland" as applied to the source of origin of an hormone is in a way misleading. By gland we understand in the morphological sense a secreting epithelial structure which may or may not beprovided with a duct. Now, since many of the in-ternal secretions so-called, have their source in organs of a non-epi-thelial nature, it is at once apparent that the term" ductless gland" if it is to be retained is to be applied to a series of organs which have in common a similar physiological function to perform, though differ-ing very widely in anatomical structure. Function then, and not structure, brings a varying group of organs together for common consideration. Knowledge concerning these most important organs has been obtained in various ways. Theexperimental method of removing an organ and noting the effects produced thereby has yielded much valuable information as has also the method of grafting. But here, and perhaps more so than in any other field of physiology, has the clinical method been of inestimable value. Comparative anatomy and histology have also been of service in making clear the proper significance of these organs. While an enormous amount of study has been given to the" ductless gland" in the last few years (indeed the papers on this one subjectnow number upward of three thousand) and a great deal of positive knowledge has been gained, there yet remain many