PROTOPLASM AND CELLS.
The ultimate elements of which the body is composed consist of masses, microscopic in size, of a living material called protoplasm. In its simplest form, protoplasm is a homogeneous looking substance, semi-fluid, without apparent differentiation of parts. It may also appear studded with fine or coarse granules, or ex hibiting a fine or coarse, more or less irregular net-work. Often it exhibits little spaces, vacuoles, filled with fluid. In the living state it possesses the power of spontaneous movement, evinced by change of form. Thus at one moment the little mass is more or less spherical, then it becomes irregular in outline by one or more processes of its substance being pushed out. Moreover, by pushing out one process in one direction and retracting another, it can change its place (Fig. 3). Now such minute masses of protoplasm are found forming the whole substance of certain micro scopic living things. The amteba of stagnant pools is such an organism, living an inde pendent life, growing by enveloping with its processes suitable particles in the water with which it comes into contact, building them up into living protoplasm like itself. The material fit for its nourishment it builds up, not at a bound but in various stages, and as a result of its activity waste substances are produced.
Thus in such a little mass of protoplasm, there are found not only the substances of which the protoplasm properly consists, namely, proteids chiefly, and also carbohydrates, fats, and salts, but also other substances, in but not of the protoplasm, lodged in the meshes of its net work or in vacuoles, some of which are in pro cess of being built up into living protoplasm, while others are the waste products of its activity. Now while there are found, in the lowest realms of animal life, organisms consist ing of nothing more than has been described, there are others similar to them, which possess a small body in the interior called a nucleus. This is spoken of as nucleated protoplasm.
When the animal body is carefully examined, in all the tissues there are found masses of nucleated protoplasm of various sizes and shapes. In all essential features they resemble
the structures described. Such bodies are called cells. In many of them the nucleus is finely granular or reticulated in appearance, and on the threads of the mesh-work may be one or more enlargements, called nucleoli. In sonic eases the protoplasm at the circumference of the mass is more or less modified, condensed, so that the appearance of a limiting membrane is produced, or cell-wall.
A cell, then, is a mass of nucleated protoplasm, the nucleus may show a nucleolus, and the cell may be limited by a cell-wall. The only essen tial thing, however, is the living ever-changing protoplasm ; and one must ever bear in mind the double process that, while it lives, goes on within the protoplasm, the process of building up lifeless into living stuff, and the process of breaking down by which waste is produced.
Moreover, the annoba we have spoken of multiplies by division, each half thus formed going free as an independent organism, in due time also dividing into two independent organ isms. If the animal body be studied in its development, it is found to originate from a single mass of nucleated protoplasm, a single cell, the ovum or egg(Fig. 4) showing both nucleus and nucleolus. From this single original cell two are formed by division, then four, and so on till a little mass of cells is produced, and from these, by further growth and development, the ani mal body with all its various tissues is evolved. This view of the development of living structures was first put forth by two Germans, Schleiden and Sehwann, in 1838, and was termed the cell-theory.
Many fully-formed tissues consist chiefly of cells, notably the liver (see p. 200). In many others the cells have been modified to form fibres, such as tendon, muscle, nerve, &c. In the blood are found bodies, the white blood corpuscles, exhibiting all the characters of the amoeba (Fig. 3).