Fig. 40.—Three kinds of cells, magnified a thousand times linear. A, a row of cilia-bearing cells. B, a single detached ciliated cell: observe the nucleus in each cell. C, a goblet-cell, from a mucous surface, producing c, a slimy secretion; d, the wall of the cell; b, the nucleus; a, the protoplasm in which the secretion c was accumulated until it burst out at the free end of the cell. D, a fat-cell; a, the nucleus surrounded by protoplasm; e, the thin layer of protoplasm enveloping the great oil drop f, which has formed within it.
[Transcriber’s Note: The original image is approximately 2¾ inches (7cm) high and 2 inches (5cm) wide in total.]
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.