[Illustration: FIG. 37.—A, cells forming soft vegetable tissue; _a_, cell-wall; _b_, protoplasm; _c_, liquid-holding cavity in the protoplasm; _d_, the nucleus. B, a pigment-cell from the frog’s skin, expanded. C, the same cell contracted. D, a nerve-cell: observe the nucleus. E, a muscle-cell stretched. F, the same contracted: observe the nucleus.] [Illustration: FIG. 38.—Copy of part of Robert Hook’s drawing of a magnified piece of cork, showing the “cells” so named by him in 1665.] 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. [Illustration: FIG. 39.—A piece of cartilage, showing the cells which have formed it embedded in the (shaded) firm substance, and connected to one another by branching processes of protoplasm.]