Immediately after its discovery in the seventeenth century, the compound microscope was applied to the study of minute plants and animals, their organs and tissues. In this connection and for this purpose the microscope has steadily improved, until perfection has almost been attained. Calculations based upon the physical properties of refracting media show that the limits of the assistance which it can give to the eye have been very nearly reached. One of the first results of the application of the microscope to the study of parts of plants was the discovery of their cellular structure. Robert Brown, looking at slices of cork, saw that its tissue is divided into compartments. It is difficult to ascertain who it was that first used the word “cell.” The resemblance of a slice of vegetable tissue or the surface view of a petal of a flower to honeycomb is so striking that the same comparison probably occurred to the mind of everyone who saw it. Further study with better instruments showed that the cells are not empty. Each cell contains cell-juice, or cell-substance, and in the centre of the cell-substance a miniature cell, the nucleus. Naturalists therefore extended the connotation of the term. A cell was no longer a space with enclosing walls; it had contents. A nucleus was invariably a constituent of the cell. The cell was regarded as an anatomical unit, consisting of a wall, cell-contents, and nucleus. In 1839 Theodor Schwann, using his microscope in the study of animal tissues, recognized the similarity between animals and plants. Animals also, he discovered, are aggregations of cells. He enunciated the =Cell Theory=. Philosophers are always ready to generalize. It is their business. Seeing that vast numbers of organisms are single cells, that they feed, breathe, divide, and reproduce their kind—in fact, carry out all the functions of life—as isolated cells, they conceived the idea that a visible plant or animal is a community of cells, each an organism in itself. As bees are units of a swarm, as men and women are units of a state, cells are units which for the sake of mutual protection remain associated in a multicellular body. The physiological or sociological aspects of this conception we shall consider shortly; but the anatomical basis of the cell theory was laid without a sufficient testing of the facts upon which it rests; or, rather, one ought to say that, although the axiom, enunciated by Virchow when he applied the cell theory to tumours and other morbid growths, _Omnis cellula a cellulâ_, holds good, the applications of the theory which certain of its later exponents have made are not necessary sequents.