could be observed. Hence the importance of the cell wall as an essential compc nent of a cell was still further diminished; and Leydig then defined a cell to be a little mass composed of a soft substance enclosing a central nucleus. But a most important advance in our conceptions of the essential structure of a cell was made when Briicke pointed out that the contents of cells not unfre- quently possessed the property of spontaneous move ment and contractility, and when Max Schultze deter mined that the contractile substance termed sarcode, which forms so large a part of the bodies of the lower animals, was analogous and apparently homologous with the contents of young actively-growing animal and vege table cells, before a differentiation of these contents into special secretions or other materials had taken place. As the term " protoplasm" had been introduced by Von Mohl to express the contents of the vegetable cell, which under goes changes in the process of growth, it was adopted by the animal histologist; and Max Schultze suggested that a cell should be defined to be a nucleated mass of protoplasm, a definition which is adopted in this article. Now, as protoplasm, whether it occurs along with a nucleus in the form of a cell, or in independent clumps or cytodes, exhibits not merely the property of contrac tility, but the power of growing and maintaining itself, it is regarded as the functionally active constituent of the cell. And thus our conceptions as to the part of the cell in which its functional activity resides have passed through three phases. In the first, the cell wall; in the second, the nucleus; in the third, the protoplasm cell contents, or cell substance, has been regarded as the active constituent, not only as regards its nutrition, but the reproduction of young cells. But though the protoplasm can of itself perform these offices, yet there can be no doubt, as Barry and Goodsir were the first to show, that the nucleus of the cell plays a part not unfrequently in the multiplication of cells by self-division. One of the most characteristic cells is the mammalian ovum. In it a cell wall exists, known as the zona pellucida or vitelline membrane; within this envelope is the granular yelk or cell contents, in the midst of which is imbedded the nucleus or germinal vesicle, which in its turn contains the nucleolus or germinal spot. The granules of the yelk are a special metamorphosis of the protoplasm cell substance. Schwann made the important generalisation that the tissues of are composed of cells, or of materials derived from cells, "that there is one universal principle of develop ment for the elementary part of organisms, however different, and that this principle is ithe formation of cells." The ovum is the primordial or fundamental cell, or germ-cell, from which, after being fertilised by the male sperm, the tissues and organs of the animal body are derived. Within the fertilised ovum multiplication of cells takes place with great rapidity. It is as yet an un settled question how far the original nucleus of the ovum participates in this process of multiplication; but there can be no doubt that the protoplasm cell contents divide, first into two, then four, then eight, then sixteen segments, and so on. Each of these segments ot protoplasm con tains a nucleus is, in short, a nucleated cell, and the protoplasm of these cells exhibits the property of con tractility. The ovum or germ-cell is therefore the imme diate parent of all the new cells which are formed within it, and mediately it is the parent of all the cells which, in the subsequent processes of development and growth, lucicia; p, protoplasm of Jffl AffifeMI or germinal spot. the animal body [ANATOMY OF are descended from those produced by the segmentation of the yelk. The process of development of young cells within a parent cell, whether it occurs in the ovum or in a cell derived by descent from the ovum, is called the endogenous reproduction of cells. But cells may multiply by a process of fission i.e., a constriction, gradually deep ening, may take place in a cell until it is subdivided into two; the nucleus at the same time participating in the constriction and subdivision. A third mode of multipli cation of cells is by budding : little clumps of protoplasm bud out from the protoplasm of the parent cell, become detached, and assume an independent vitality. If a nucleus differentiates in the interior of such a clump, it becomes a cell; if it remains as a mere clump of proto plasm, it is a cytode. These various methods of multiplication are all con firmatory of Schwann s generalisation of the descent or derivation of cells from pre-existing cells. But as the nucleated cell, either with or without a cell wall, is not, in the present state of science, regarded as the simplest and most elementary unit capable of exhibiting vital pheno mena, and as these phenomena can be displayed by indi vidual clumps of protoplasm, without the presence of a nucleus, some modification of the doctrine, as regards the formation of the tissues from nucleated cells, seems to be necessary. For, although there can be no doubt that all the tissues are mediately derived from the ovum or funda mental cell, and that most of the tissues are derived directly from nucleated cells, yet there is reason to think that a differentiation of a cytode clump of protoplasm into tissue may take place, so that the direct formation of such a tissue would be, not from a nucleated cell, but from the more simple cytode. Hence a more comprehensive gene ralisation, to which observers have gradually been led from the consideration of numerous facts, has now been arrived at, that the tissues and organs of the body, what ever may be their form and composition, are formed of protoplasm, or produced by its differentiation; and that the protoplasm itself is derived by descent from the proto plasm substance of the primordial germ-cell. Some, in deed, have contended that protoplasm, cells, and their derivatives can arise by a process of precipitation or aggregation of minute particles or molecules in an organic infusion, and that living matter may be thus spontane ously generated. But the evidence which has been advanced in support of this hypothesis is by no means satisfactory or conclusive, whilst the correctness of the theory of the direct descent of protoplasm from pre-exist ing living protoplasm is supported by thousands of observations made by the most competent inquirers. In the process of conversion of protoplasm into the several tissues, there takes place a differentiation of form and structure (i.e., a morphological differentiation), and of composition (i.e., a chemical differentiation), as the result of which a physiological differentiation is occasioned, whereby tissues and organs are adapted to the performance of special functions. Hence arise the several forms of tissue which occur in the human body and in the higher animals. Many of the tissues consist exclusively of cells which present in different parts of the body characteristic modifications in external configuration, in composition, and in properties, as may be seen in the fatty tissue, pigmentary tissue, and epithelium. Other tissues, again, consist partly of cells, and partly of an intermediate material which sepa rates the constituent cells from each other. Here also the cells present various modifications; and the intermediate material, termed the matrix or intercellular substance, varies in structure, in composition, and in properties in the different textures, as is seen in the connective,
cartilaginous, osseous and muscular tissues
