The body of man, a bird, a lizard, an oak tree, and many other animals and plants, may usefully be compared to such mosaic figures; for, just as the mosaic figure has its structural unit, the little bit of glass or stone, called the mosaic; so the bodies of men, birds, and other creatures may be looked upon as infinitely complex figures formed of minute mosaics called cells.
The groups of minute mosaics or cells that make up the bodies of animals and plants differ profoundly from the mosaics that form the figures of the dead Emperor and his companions, inasmuch as each mosaic or cell of the animal or plant body is so small as to require the microscope to reveal it; also each mosaic of the living animal or plant is a living mosaic or cell, in that it can absorb food, digest it, assimilate it, grow, and multiply in numbers. Cells, then, are the morphological or structural units which compose the bodies of all living creatures.
All animals and plants begin life as single cells, which, in the vast majority of cases, are microscopic in size. Those that remain single cells, and dissociated throughout life, are called Unicellular Animals (Protozoa) and Plants (Protophyta), and are to be seen mostly by the microscope alone; but those which, by multiplication and growth, form large numbers of cells that remain associated together as in the body of a bird or lizard, are called Multicellular Animals (Metazoa) or Plants (Metaphyta).
A cell ([Fig. 1]) is a nucleated lump of protoplasm, or cytoplasm, and most often of microscopic size and more or less covered on its exterior by, and holding in its interior, various products and formations resulting from its activity, which are called metaplasm. Since the protoplasm of a cell, under the microscope, presents a superficial resemblance to a minute speck of that jelly-like substance (albumen) which forms the white of an egg, it is often called an albumenoid substance. But it is very misleading to use such an expression, for protoplasm is not a single chemical substance of great complexity; but it is rather composed of a large number of different chemical substances of great complexity. Many of these substances, it is true, are albumenoid in character. The same is true as to the chemical complexity of the nucleus, which is a physically and chemically differentiated part of the protoplasm.
The protoplasm contains certain globulins, and also albumins and peptones; it also contains large quantities of nucleo-albumins, with other substances. The nucleus not only contains these same substances, but also nuclein and nucleo-proteids. It is important to state that nuclein consists of an albumin and nucleic acid.
Fig. 1.—Diagram of a Cell, highly magnified.
The protoplasm, structurally, is made up of threads forming a complex, sponge-like substance, or reticulum, called spongioplasm; and in the meshes of the spongioplasm is a more or less fluid-like substance known as hyaloplasm: suspended in the hyaloplasm are various kinds of living bodies known as plastids, besides various products resulting from the activity of the protoplasm and which are designated metaplasm.
In many cells the protoplasm has formed on its periphery a layer of metaplasm which is frequently called a cell-wall. This cell-wall prevents amœboid movements of the protoplasm, and a cell possessing it is said to be encysted.