that is, they contain a central kernel, the nucleus, which is more dense than the surrounding protoplasm, and of a slightly different chemical composition. These bodies, which circulate among the tissues with the blood forming a part of it, manifest independent movements, thrusting out and drawing in portions of their mass in true amœboid fashion; they devour solid particles of matter which come in their way—their smaller comrades, the red corpuscles, not always escaping their voracity. This they do by flowing around and inclosing them, as already described. They have also been observed by Klein to multiply by division, like the monera. The white blood-corpuscle, identical, apparently, with the amœba, which may be found in the standing water of pools attached to the surface of leaves, and in many other similar situations, is a true cell—the morphological unit from which all organisms, whether low or high, originate, and by whose multiplication, development, and differentiation, all the tissues of their bodies are produced.
The history of the growth and development of every animal—whether moner, mollusk, or man—is a history of cell-multiplication and cell-differentiation; and the most highly endowed individual of them all possesses no property, no faculty, no power, which is not at last foreshadowed in the formless, structureless, protoplasmic cell from which they are all alike derived. Is the nervous tissue of man in the highest degree irritable and automatic—that is, sensitive and self-acting? So is protoplasm, though in an almost infinitely less degree. Is muscular tissue eminently contractile, serving for the production of the varied and complicated movements of all parts of the body? Protoplasm is also capable of slight spontaneous motions of its entire mass. Are the various glands of the body actively secretory and excretory? So is protoplasm within the narrow limits of its chemical necessities. Equally, also, with the highest tissues and organisms, it reproduces its kind.
The complex body of any one of the higher animals may, then, be considered as consisting of certain tissues, each of which has not only been derived from protoplasm but each of which corresponds, in its perfected function, to some one of the fundamental properties of protoplasm, to the special manifestation of which it is devoted for the benefit of the organism as a whole, on the important principle first spoken of by Milne-Edwards as the physiological division of labor. Division of labor among the tissues, however, as among the members of a community, has its limits. While every tissue has some leading quality, some special function, developed to the highest degree in the interests of the organism as a whole (contraction in muscle-tissue, secretion in gland-tissue, and so on), yet each tissue retains in its own private interests, as it were, vestiges of all the other protoplasmic properties belonging to their common ancestor. Hence, all the tissues are assimilative to the extent of keeping up their own nutrition; all are