[Illustration: FIG. 36.—Simple “cells,” consisting of naked protoplasm, changing shape and taking in solid food particles. A, is a series of four successive changes of shape of a fresh-water animalcule, the proteus or amœba; B, is a similar series of three views of a separate creeping kind of corpuscle found in the blood and lymph-spaces of animals, and called a “phagocyte.” It is also said to be “amœboid,” from its resemblance to the amœba or proteus-animalcule. B, is from the blood of the guinea-pig. It is not a parasite, but one of the various kinds of cells which build up the animal body, and are derived from the single original egg-cell (see Fig. 31) by continued division. The three drawings show three changes of shape occurring in the same “phagocyte” in a few minutes. It is engulphing a fever-producing blood-parasite, a spirillum, marked _a_, into its soft, slimy protoplasm, to be there digested and destroyed. In the same way the amœba, A, is seen in four stages of engulphing the vegetable particle, _a_. In the fourth figure the letter _b_ points to water taken into the amœba’s protoplasm with the food-particle _a_. In all the figures, _c_ points to the “vacuole” or liquid-holding cavity, which bursts and re-forms in A; the letter _d_ points to the cell-nucleus.] 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.