between an animal and vegetable cell; since the vegetable cell has a cell-wall, while the 'animal cell has none. An animal cell may, therefore, be defined as 'a mass of protoplasm possessing a nucleus and sometimes a nucleolus.' PRODUCTION OF ELEMENTARY CELLS. The It was Schwann who first made the important general statement, that all the tissues of the animal body were composed of or derived from cells. The cells of the animal body are derived from the fertilised ovum. mammalian ovum differs from the elementary cells already described in possessing a distinct cell-wall, which is called the Zona pellucida, or vitelline membrane; but it resembles them in being composed chiefly of a granular, protoplasmic material, the yelk, within which again is the germinal vesicle and germinal spot, which correspond respectively to the nucleus and nucleolus of the ordinary elementary cell. The elementary or embryonic cells are produced from the fertilised ovum by a process of cleavage or segmentation. The germinal vesicle and germinal spot disappears, and the protoplasmic material, shrinking slightly, divides into two equal parts. These segments subsequently divide into two, and this division, in multiple proportion, continues until the entire yelk is split into a number of small segments, each consisting of protoplasmic material and possessing a nucleus. In this way the ovum is the parent of all the new cells which are formed within it, and the progenitor of all the cells which are descended from those which result from the segmentation of the yelk. The following is a brief outline of the various forms of cell genesis: 1. Fission. In this form of development a cell becomes constricted, and, owing to the gradual deepening of the constriction, subdivides into two parts, the nucleus also participating in the subdivision. 2. Budding. When the multiplication takes place by budding, little processes of protoplasm bud out from the parent cell, and, subsequently becoming detached, assume an independent existence. 3. Endogenous reproduction is a form of internal budding, by which the young cells are developed within the parent cell till the original cell is broken up. This last form is well exemplified in the production of the elementary cells from the ovum in the manner described above. PROTOPLASM. The basis of vital activity; is a complex substance, from which can be obtained, by analysis, examples of the albuminous, fatty, and starchy groups, with a variable quantity of salts. It is usually viscid, varying much in consistency,—sometimes being semi-fluid, at other times being an exceedingly viscid, coherent material. When exposed to a temperature of about 130° F. it undergoes heatstiffening and dies, undergoing a kind of coagulation. Protoplasm is met with in many varied forms; it occurs inclosed in tubular sheaths (as in muscle), or as granular masses of variable form (cells) with nuclei. Such contractile masses form the whole substance of the body of many of the lower forms of animal life, as the Amoeba, or the soft portions of other animals. It occurs in colourless blood-corpuscles, and also in the analogous corpuscles of areolar tissue, in the lymph corpuscles, and in mucus and pus in the higher animals. The way in which cells are associated together to form different tissues and substances depends to a great extent upon the matrix or intercellular substance, and upon the relation the cells bear to it. I. The intercellular substance may be fluid, as in blood, which may be regarded as the simplest form of tissue. 2. It may consist of a soft, semi-gelatinous material, as in mucous tissue. 3. It may be firm and solid, as in cartilage or bone. 4. Lastly, it may be very slight in amount, as in epithelium, where it seems merely to cement the cells together. The tissues of the human body have been divided, according to their functions, into two great classes; namely, those which have some active, vital function to perform, and those whose function in the organism is of a purely passive nature, and which serve to connect and support the various parts of the body. Those tissues which belong to the former class are called active tissues, and those belonging to the latter class are called passive tissues. The tissues which belong to the two |