Without doubt, the greatest and most important statement which can be made about living things is that they are either separate minute particles of living matter or (more commonly) are built up by thousands of such minute particles which have in each individual animal and plant originated from a single such particle (the fertilised germ), by its division into two, and the subsequent division of these two each into two, and of the four so produced each into two—and so on, until by repeated division into two, millions of corpuscles, hanging together as one mass, are the result.

The particles of living matter are spoken of as “cells” for a very curious reason, to which I will revert. The living matter is called “protoplasm” (primitive or fundamental slime). A “cell” in the language of microscopists means a corpuscle or more or less rounded or irregularly shaped particle of protoplasm. Cells commonly vary in size from 1/5000th to 1/200th of an inch in breadth, and may be much larger. Protoplasm—the living substance of “cells”—is a slimy body, almost liquid, but yet tenacious. It is transparent, but clouded by fine granules, and can often be seen with a very high power of the microscope to consist of more and of less liquid matter, intermixed like an emulsion. It often has within it large cavities filled with liquid, and also often oil drops; in other cases hard concretions or coarse granules. But apart from other things, the protoplasm of a “cell” always contains within it a special, firmer, and denser part, enclosed in an enveloping coat or skin. This dense body is the “nucleus,” or kernel, and is of the very greatest importance in the chemical changes and movements which constitute the life of the cell. It is usually spherical, and in the living state often looks clear and bright. All cells, whether they are found building up the bodies of plants and animals like so many living bricks, or living freely and singly as animalcules, have the essential structure just described—a semi-liquid yet tenacious material enclosing a globular firmer body, the nucleus.

How did these viscous nucleated corpuscles come to be called “cells”? It was in this wise. At the end of the seventeenth century Dr. Robert Hook, secretary of the Royal Society, published a beautiful book of folio size, entitled Micrographia. In this he pictured various minute insects and various natural products as seen under his microscope. Among the objects figured and described was a piece of cork (Fig. 38). Hook showed that it was built up of a number of empty, air-holding, box-like chambers, less than the hundredth of an inch in length, and these he called “cells,” comparing them to the “cells” of the bee’s honeycomb. Later observers found that this “cellular” structure was very common in plants—but it was not until more than a hundred years later that it was observed that the “cells” which build up the soft stems and leaves of plants are not empty or merely air-holding, but contain a liquid or viscid matter. Robert Browne, a great botanist, who lived within the memory of some of our older naturalists, first observed and described the “nucleus,” or kernel, within the cells of some lily-like plants, and gave it that name (Fig. 37 A, d). About the thirties of last century, by aid of improved microscopes, a structure like that of the vegetable “cell” and its “nucleus” was discovered in some animal materials, or “tissues,” as they are termed—for instance, in cartilage (Fig. 39). The word “tissue” is applied to each of the various layers and masses, such as epiderm, fibrous tissue, muscle, nerve, cartilage, bone, which can be distinguished in an animal body and separated from one another, just as we may separate the “tissues” of a man’s clothes—the leathern, woollen, silken, cotton, linen: the cords, laces, threads, and pads or stuffing. The full meaning of this existence of “cells” or “cellular” structure in the tissue of plants and animals only gradually became evident. A very remarkable discoverer, Professor Schwann, of LiÈge (with whom when he was an old man I spent an afternoon a great many years ago), was the first to grasp the great facts and to put forward what has been ever since called “the cell theory” of animal and vegetable structure and life.

Schwann, in 1836, showed that the important thing about a “cell” is not the box or cell-wall so much as the viscid contents and the nucleus. But the name “cell” was (strangely enough) retained for the contents, even when the box-like chamber was absent—much as we speak of “a bottle of wine,” meaning the contents of the bottle, and not the glass vessel holding it. It was shown that the box-like case or cell-wall (the original “cell” of Hook) is actually formed by the living nucleated plasm or viscid matter within it, just as a snail forms its shell, by the separation or “secretion” of a dead, firm, chemical deposit on its living surface. Schwann showed that all—not merely special exceptional instances, but all—the tissues of plants and of animals are built up by nucleated cells, the cell-wall being often not hard and box-like, but soft, gelatinous, irregular in shape, and sometimes very thin, sometimes very thick. Every living cell is thus surrounded by the chemical products of its own activity, or may deposit those products within itself as in the goblet-cell and the fat-cell seen in Fig. 40, C and D, and these products differ in different tissues. The cells of a tissue, using the word to mean the soft nucleated particles or corpuscles of protoplasm or “cell-substance,” must be regarded as the microscopic living “weavers” or makers of the tissue. The cells in one tissue may form a honeycomb of boxes; in another a jelly-like mass or a fibrous network, with the cell-substance scattered as nucleated particles in it (Fig. 39). Or the cells may be elongated and contractile (Fig. 37, E, F). They may be more or less fused with one another, as in flesh or muscular fibre; but we can always recognise the presence of the individual cells under the microscope by their distinct and separate “nuclei.”

Schwann’s most important conclusion from this universal presence of soft corpuscles of cell-substance, each with its globular nucleus, in all the tissues and most varied parts of animals as well as plants, was that the life of a living thing, the chemical and physical changes which go on in it from birth to death, consist in chemical and physical changes in each of these microscopic, nucleated bodies, and that the life of the whole animal or plant is the sum of the lives of these microscopic units. If we wish to know more about the real nature of the growth and activities of living things, said Schwann, we must thoroughly study and ascertain the chemical and physical changes, and the properties of the cell-substance in all the different varieties of tissue. That is the celebrated “cell-theory” of Schwann. And this examination of, and experiment with, the cells of all kinds of tissues of plants and animals has been going on ever since Schwann made his historic statement more than seventy years ago. The branch of science called “histology” is the outcome of that study.

Microscopes have been immensely improved since Schwann wrote, first in England by the father of the present Lord Lister, then later in Germany by AbbÉ and Zeiss, of Jena. A variety of methods have been devised for making the “cells” in thick, solid tissues visible. Very thin sections—thin enough to be transparent—were at first cut from the fresh tissues, and examined by transmitted light. This did very well in a rough way, but better results were obtained by hardening the tissues in alcohol or chromic acid, when wonderfully fine sections could be cut and rendered translucent by soaking in varnish, in which they were preserved for study with the microscope, between two plates of glass. The sections were stained with various dyes, such as carmine, log-wood, the aniline dyes, etc., and it was found that the nuclei of the cells and the granules and fibres both in the minute cells and in the surrounding substance manufactured by them, could be distinguished more clearly by means of their differing affinity for the dyes. And whilst endless section-cutting and staining and careful drawing and record of the structure discovered, was proceeding in hundreds of laboratories—other observers especially devoted themselves to the difficult task of seeing the cell-substance or protoplasm and its nucleus under the highest power of the microscope, whilst still alive! It would seem a hopeless task to examine with a high-power microscope the cells (less than a thousandth of an inch broad) inside the solid stem or leaves of a plant or of an animal’s body without killing the plant or animal and the cells of which they consist. As most of my readers know, the front lens (or “glass”) of a high-power microscope has to be brought very close indeed to any object in order to bring it into focus—as near as the one twenty-fifth of an inch. Then the object examined must be very small and transparent, in order that the light may pass through it, as through the slide-picture in a magic lantern, and so form a clear, well-defined picture in the focus of the microscope, where the eye receives it.

Fortunately, there are some facts about living cells or corpuscles of protoplasm which enable us to examine living cells, in spite of these difficulties. In the first place, there are a whole host of minute animals and plants—of many different kinds—which consist of only one cell or nucleated corpuscle of protoplasm (Fig. 36 A); they are transparent, abound in fresh water and sea water, and can be searched for with the microscope in a drop of water placed on a flat glass plate and covered with a specially thin glass slip. Many of these have been studied for hours—and even days—continuously, and the remarkable internal currents and movements of their viscid “protoplasm,” its changes of shape, its feeding and growth, and the details of the process of division into two—by which it multiplies—have been ascertained, as well as the action upon it of light, heat, electricity, and mechanical shock, and of all sorts of chemical substances, carefully introduced beneath the cover-glass. A second fact of great importance is that the “cells” or protoplasmic corpuscles, which build up a complex plant or animal, do not die at once when the plant or animal “dies,” that is to say, the animal or plant may be “killed” and fine bits of transparent tissue removed from it and placed beneath the microscope, where, with proper care, the cells may be kept alive for some time. The hairs of many plants are strings of transparent “cells,” or boxes, containing living, streaming, active protoplasm. These hairs can be cut off, and the cells will remain alive for a long time whilst they are under the microscope (see Fig. 15 bis). The transparent wall of the eye—called the cornea—can be removed from a frog after it has been killed, and the still-living cells in the delicate glass-like tissue can be studied with the highest powers of the microscope, and give evidence of their life by their movements and other changes. Most convenient and important for this study is the blood—for there the cells are loose, floating in the liquid. The cells in a minute drop of human blood can be kept alive for hours, if the glass slide is kept warm, as it easily can be, and I have seen the cells in a drop of frog’s blood (skilfully treated) still alive, and exhibiting active movements, a fortnight after the frog, from which the drop of blood came, was dead and buried. These floating, moving cells of the blood are the “phagocytes,” which engulf and digest disease germs and other particles (Fig. 36 B). Other more numerous cells of the blood are the oxygen-carriers, or red corpuscles, which do not show any movements or changes of an active kind whilst alive.