cause which produces the cells of the dorsal cord could not be different from the one that gives origin to the vegetable cells." The two men went together to the amphitheatre of anatomy to examine the nucleuses in question, and Schleiden recognized a complete resemblance between them and the nucleuses of the cells of plants. "From that time," Schwann continues, "all my efforts were directed to finding proof of the pre-existence of the nucleus in the cell." And he goes on to tell how his views were confirmed as his researches advanced. At the time Schwann thus undertook to show that all the organs are of cellular origin, the structure of most of them was very imperfectly known. The application of the microscope to researches in animal histology was of recent introduction, and everything was to create. Schwann did not shrink from the tremendous task which opened up before him; and what he had done first for the cartilages and the dorsal cord, he tried in succession for all the other bodily tissues; and in all he had the joy of seeing his idea confirmed.
Schwann came upon many new discoveries in the course of these investigations. He first compared the egg to a cell, and recognized cells in the globules of the blastoderm; described the stellar pigmentary cells, the layers of the nail, the development of feathers, the nucleuses of the prisms of the enamel, those of the smooth and striated muscles, the fibers of the dental pulp, the cells destined to be transformed into fibers of the crystalline, etc. He called attention to the envelope of the nervous fibers which bears his name as the sheath of Schwann—all of which discoveries have been confirmed by modern research armed with its more perfect technic and superior instruments. The theory of the cell as the primordial element of all the tissues was hereafter to serve as the Ariadne's thread to the numerous investigators who devoted themselves to the study of morphology, ' and was to help them explain the infinite variety of organic forms. It gave a definite purpose to the application of the microscope to investigations in anatomy and physiology. It was the foundation of modern physiology? and all the morphological progress accomplished during nearly the past half-century has grown out from it. Except for its having familiarized the conception of the constitutional unity of living matter, and having declared the principle that every cell is the product of another cell, the doctrine of selection and descent could not, in the opinion of Edward Van Beneden, have gained ground. Its salutary influence in pathological anatomy and the advance of physiology was immediate and great. Acting in another direction, it put an end to the theory of a special vital force, which was in full sway when it was first promulgated, and raised up that of physico-chemical action, which has taken its place. How was it possible to reconcile the notion of cellular
