The “cell theory” is the doctrine that the bodies of all animals and plants consist, either of a cell, or of a number of cells, and their products; and that all cells proceed from cells, as expressed in the phrase _omnis cellula e cellula_: a doctrine foreshadowed by Kasper Freidrich Wolff, who died in 1794, and by Karl Ernst Von Baer (born 1792.) It was established in botany by Schleiden in 1838, and in zoology by Theodor Schwann about 1839. Its complete form, including the ovum, as a simple cell, also, is the basis of the present state of the biological sciences.--_Cent. Dic. 1, p. 879, col. 1._ Sec. 6. Protoplasm Protoplasm is an albuminoid substance, ordinarily resembling the white of an egg, consisting of carbon, oxygen, nitrogen, and hydrogen in extremely complex and unstable molecular combination, and capable, under proper conditions, of manifesting certain vital phenomena, as spontaneous motion, sensation, assimilation, and reproduction, thus constituting the physical basis of life of all plants and animals; sarcode. It is essential to the nature of protoplasm that the substance consist chemically of the four elements named (with or without a trace of some other elements); but the molecule is so highly compounded that these elements may be present in somewhat different proportions in different cases, so that the chemical formula is not always the same. The name has also been somewhat loosely applied to albuminous substances widely different in some physical properties, as density or fluidity. Thus the hard material of so-called vegetable ivory and the soft body of an amœba are both protoplasmic. The physiological activities of protoplasm are manifested in its irritability, or ready response to external stimuli, as well as its inherent capacity of spontaneous movement and other indications of life; so that the least particle of this substance may be observed to go through the whole cycle of vital functions. Protoplasm builds up every vegetable and animal fabric, it is itself devoid of discernible histological structure. It is ordinarily colorless and transparent, or nearly so, and of glairy or viscid semi-fluid consistency, as is well seen in the bodies of foraminifers, amœbæ, and other of the lowest forms of animal life. Such protoplasm (originally named sarcode) when not confined by an investing membrane, has the power of extension in any direction in the form of temporary processes capable of being withdrawn again; and it has also the characteristic property of streaming in minute masses through closed membranes without the loss of the identity of such masses. An individuated mass of protoplasm, generally of microscopic size with or without a nucleus and a wall, constitutes a cell, which may be the whole body of an organism, or the structural unit of aggregation of a multicellular animal or plant. The ovum of any creature consists of protoplasm, and all the tissues