In biology a cell is defined, first, as the fundamental form-element of every organized body. Secondly, as a nucleated, capsulated form element of any structure or tissue; one of the protoplasmic bodies, which build up an animal fabric; a body consisting of cell-substance, cell-wall and cell-nucleus, as bone-cell, etc. (Cent. Dic. 1, p. 878.) The body of every animal and plant is made of cells; and each cell is composed of many atoms. For a full discussion of “The Atomic Theory,” see Encyc. Brit. 3, pp. 36-49, (9th ed.); New Int. Encyc. 13, pp. 683-685. Sec. 5. Cells and Cell Theory In Biology, the word “cell” denotes the fundamental form-element of every organized body. It is a bioplastic mass of protoplasm, varying in size and shape, generally of microscopic dimensions, capable, under proper conditions, of performing the functions of sensation, nutrition, reproduction and automatic or spontaneous motion, and constituting in itself an entire organism, or being capable of entering into the structure of one. Such a cell, as a rule, has a nucleus and is usually also provided with a wall or definite boundary; but neither cell-nucleus nor cell-wall necessarily enters into its structure. In ultimate morphological analysis, all organized tissue is resolvable into cells or cell products. See “Protoplasm,” and “Cell Theory,” infra. Specifically, the word “cell” denotes a nucleated capsulated form-element of any structure or tissue one of the independent protoplasmic bodies which build up an animal fabric. A body consisting of cell substances, cell-wall and cell-nucleus, as bone cells, cartilage-cells, muscle-cells, nerve-cells, fat-cells, cells of connective tissue, of mucous and serous membrane, etc., of the blood, lymph, etc. This is the usual character of cells in animals, and is the ordinary technical anatomical sense of the word. “However complicated one of the higher animals or plants may be,” says Huxley, “it begins its separate existence under the form of a nucleated cell.”--Huxley, _Anatomy Invert. An. p. 19_. See Haeckel, Ev. Man, chap. 6. “_Ovum and amœba_,” pp. 36-50; Spencer, _Principles, Biology, Index, “Cell,”_ 2 p. 630; Romanes, _Darwin, etc._, 1, pp. 104-134; _Encyc. Brit. 12_, pp. 5-10, “_Histology_;” _New Int. Encyc. 4_, p. 400. Professor McMurrich, of the University of Michigan, says: “It has been estimated that the number of cells entering into the composition of the body of an adult human being is about twenty-six million five hundred thousand million.” (McMurrich, _Development, Human Body_, p. 18.) This number is equivalent to twenty-six and a half trillions.