THE author has become convinced, by several years intimate intercourse with students of medicine, that their acquaintance with the subjects he has endeavored to include in this little volume would be facilitated, if the views, which are now taught and scattered throughout the often expensive works of their authors, were collected in a convenient form for study and reference. Taking it for granted that a knowledge of this subject, is of fundamental importance in its bearing upon the study of physiology and pathology, and stimulated by the frequent inquiries of students for an appropriate source of information, he has prepared what he now submits to them. He has sought to obtain a continuous history of the evolution of the cell doctrine up to its present state, without embarrassing his pages with a large number of isolated facts. He has attempted, however, to secure a completeness, and to make the work useful to physicians and others engaged in research, by careful references, and the addition of a bibliography, which he has sought to make accurate and extended. Some authors may have been overlooked such the writer cordially invites to send him references to their own papers, or to those of others they believe to have a bearing upon the subject. THE idea that animals and plants, however complex their organisation, are really composed of a limited variety of elementary parts, constantly recurring, was appreciated by Aristotle, while it appears to have been even more clearly conceived by the acknowIedged father of medical science, Galen.....