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Cell. Zoology. A cell is the simplest structural element entering into the composition of all living beings. Animals may consist of one separate cell, as in the Protozoa; or of many cells, as in all Metazoa. A cell consists of two constituents, protoplasm and a nucleus, to which a cell wall is usually added. The protoplasm is really the essential part of a living cell. It is usually in two distinct layers, a clear, firm ectoplasm and a more fluid, granular endoplasm, containing food and oil materials. The nucleus is composed of a material known as nuclein or chromatin, arranged in a spiral or net, or as scattered granules, between which is the "nuclear fluid." The nucleus is an essential constituent of a true cell; when it is absent the structure is known as a "cytode." The simplest form of cell envelope is a mere hardening of the external layer of protoplasm produced by its dehydration; this is seen in some of the Amoebae. In higher forms it is a definite cuticle, which may be often protected by a shell of varied composition. Among the Protozoa there are various types of cells, for which see Protozoa. Among Metazoa cells are still more varied, and build up all the complex tissues of higher animals. It is comparatively rare for the cells of animals to be limited externally by the hard, firm envelope of cellulose typical of plant cells; but in many cases it is impossible to say whether the cells are to be regarded as animals or plants: e.g. the "amoebulae," which form one stage in the life history of the Mycetozoa, or the "flagellulae" of Proteomyxa.

Botany. If we cut a cross-section of the young stem of a herbaceous plant and examine it even with a low power of the microscope, we see a mesh-like, or netted, chambered structure. A longitudinal section will be similar, though the chambers or cavities, filled with liquid contents, may be somewhat elongated. Robert Hooke, in 1667, compared this structure to a honeycomb, and first gave the name of cells to these chambers. The labours of Malpighi and Grew (1670) added much to our knowledge of the internal structure of plants; but, as was natural, these early observers took the solid wall of the cell to be the first-formed and most important part. Caspar Wolff, in the eighteenth century, maintained that the bodies of living beings are composed of minute constituent units all alike in early life and gradually differentiated into tissues; and in 1755 Rosel von Rosenhof, in his account of the "Proteus animalcule" (Amoeba), gave the earliest description of what we now know as living protoplasm (q.v.). In 1772 Corti described the rotation of the contents of the cell in Chara (q.v.), and Meyen, that in Vallisneria (q.v.) in 1827. In 1835 Johannes Muller pointed out the existence of cells like those of plants in the notochord of vertebrate animals, and in the same year Dujardin carried out his important researches into that substance in the Foraminifera, for which he proposed the term "sarcode." Robert Brown's discovery of the nucleus in vegetable cells led to the recognition of the importance of that structure, and in 1838 Schleiden traced back the life-history of every plant to a single nucleated cell, and referred all the vegetable tissues to the cellular type. A year later Schwann extended this cell-theory of structure to animals, and in 1846 Hugo von Mohl discriminated the "tough, slimy, granular, semi-fluid" protoplasma, or first-formed substance, from the watery cell-sap. It was found, however, that animal cells and many free-swimming vegetable cells (Zoospores) were destitute of the cell-wall, so characteristic of ordinary plant-cells; that the cells of cork and wood, which had lost their protoplasm, had ceased to have any active vital functions; and in fact, as pointed out by Max Schuttze, between 1854 and 1861, that, whilst Dujardin's animal sarcode is practically identical with Von Mohl's vegetable protoplasm, this substance is, as Professor Huxley termed it, "the physical basis of life," and we must replace the original idea of a cell, as a closed sac of membrane with a nucleus and fluid contents, by that of a unit mass of nucleated protoplasm. This botanists now generally term a primordial cell. In most cases it soon forms for itself a cell-wall, or becomes "encysted." In a number of the lowest plants single cells are capable of leading an independent existence, and perform all the vital functions, reproducing themselves either by bipartition, by sending out buds or even by conjugation. The "daughter-cells," resulting from one or more bipartitions remaining united, form a simple transition to colonies (caenobia), cell-filaments or tissues, from unicellular plants, that is, to multicellular ones. To some of the lower plants, however, no cell-theory can be applied without violence. The unencysted Myxomycetes (q.v.), and the large alga Caulerpa (q.v.), which has rhizoids, stems, and leaves, but no internal cell-partitions, are cases in point. In a typical young plant-cell the wall is thin and is completely filled by the contents, whilst the nucleus appears large relatively to the protoplasm. As the cell gets older the wall becomes thicker, the protoplasm becomes "vacuolated" by drops of watery "cell-sap," and the nucleus, remaining comparatively unchanged, appears relatively smaller. When the cell is mature, the cell-sap has often pushed the protoplasm to the sides, so that it forms a mere film, once termed the primordial utricle. The cell-wall consists mainly of cellulose (q.v-), but is possibly in all cases permeated by protoplasmic threads. In many of the higher plants the cavities of cells communicate by perforations. The cell-wall may, by saturation with lignin, cutin, or mucin, compounds richer in carbon than is cellulose, become converted into wood, cork, or mucilage respectively. The nucleus, apparently present at some time in all vegetable cells, is undoubtedly of great physiological importance. It generally divides before the cell undergoes division: if, in some algae, it escape with some protoplasm from a broken cell, it may give rise to a new plant; and, as long as it remains, some protoplasm adheres round it. It has a delicate "nuclear wall," and differs from the protoplasm in its composition, apparently containing more phosphorus. The cell-sap is mainly water, but will contain sugar, dextrine, inuline, acids, or any other soluble substances that may be present in the cell. Besides the protoplasm, nucleus, and cell-sap and these soluble and, therefore, invisible substances, the cell may contain various granular bodies, such as aleurone-grains (q.v.), starch-grains (q.v.), and plastids, of which the principal are chloroplastids, or chlorophyll-corpuscles (q.v.), leucoplastids (q.v.), and chromo-plastids, or granules with less soluble colouring-matters. Needle-like or other crystals of calcium-oxalate or carbonate may also occur. [Raphides.] Vegetable cells range in size from .001 millimetre up to a length of nearly two inches, as in the hairs of cotton.

Physiology. The human body with all its complexity of structure takes its origin from a single cell; every tissue of which the body is composed is made up of cells (more or less modified) the descendants of this original ovum. An idea of what a cell is may be obtained from a brief consideration of the mammalian ovum. It is a more or less spherical, semi-transparent, granular-looking body, from 1/100 to 1/200 inches in diameter. The substance of which it is composed is called protoplasm (q.v.), it is limited externally by a membrane called the zona pellucida, and contains, embedded within it, a distinct globular mass, known as the germinal vesicle. Substituting for the special term germinal vesicle the word nucleus, and neglecting the outer limiting membrane, we arrive at the conception of a typical animal cell.

The modifications such nucleated masses of protoplasm undergo in the making of the fully-developed human body are innumerable. In epithelium (q.v.) are found masses of cells in close juxtaposition to one another, as on the surface of the skin and mucous membranes; in the connective tissues (q.v.) the cells are cut off from immediate contact with one another by a development of intercellular substance; in muscle and in nerve the cells undergo remarkable modifications, so that the origin of such structures from cells is far from obvious at a first glance. To Theodor Schwann is due the demonstration of the cellular origin of all animal tissues; he showed that hairs, nails, and other (till his time imperfectly understood) constituents of the body, were composed of altered cells. The "cell theory" which Schwann thus put forward did not merely insist on the fact that the cell was the structural or morphological unit, it hinted at the feasibility of arriving at a just idea of the working of the body, by a study of the properties of cells, in other words it constituted the cell the physiological unit of the animal body. Virchow described the phenomena of disease from this point of view in his celhelar pathology; to him is due the famous phrase, "Omnis cellula e cellula" - i.e. no cell originates save from a pre-existing cell.

From a structural standpoint it is noteworthy that certain modifications have been made in the idea of a cell. To the botanists is due the discovery of the cell, and hence the insistence in early times upon the cell-wall. Animal cells for the most part have no well-marked outer limiting membrane, and so the nucleus came to assume the first importance. Nowadays it is the protoplasm that commands chief attention in the physiological conception of a cell; and with this change of view comes about an alteration in the attitude of physiology towards the cell theory. It is to a study of the physical and chemical properties of protoplasm, rather than to the particular structural features of individual cells, that the modern science addresses itself. In fact, the modern physiological unit is a limited mass of protoplasm, and not an organised cell. Hence the questions of movement, power of responding to stimuli, and nutrition, will be referred to under Protoplasm; a word may, however, be said here with regard to the reproduction of cells.

Cells may give rise to new cells by budding off a portion of their substance (gemmation as it is called), by free cell-formation, or by division. The last-named method alone occurs in the human body. This division may be direct or indirect. In direct division no complex nuclear change precedes the splitting up of the cell, in indirect division the change does occur, and to it the terms karyokinesis and karyomitosis, have been applied. To understand what is meant by these terms, it must be premised that the nucleus contains a network of fibrils. These fibrils are coloured by a large number of reagents which leave the inter-fibrillar substance unstained. Hence the name Chromatin is applied to the stain-retaining network, the interstitial substance being called Achromatin. By appropriate treatment of dividing cells it has been shown that cell division is very generally preceded by a complex set of rearrangements of pattern in the nuclear network. To the figures formed by the fibrils, such terms as rosettes, wreaths, spindles, stars, etc., have been applied, and the whole process has been called karyokinesis (karyon, a kernel, nucleus, kinesis, motion; or karyomitosis, mitos, a, fibril). Within the nucleus, rounded bodies, termed nucleoli, can sometimes be seen. Their exact relation to the network is doubtful. Some regard them merely as thickened portions of the fibrils, others as distinct bodies lying free in the interstitial substance. The chemical composition of nuclei has been investigated and a substance called nuclein has been isolated and described.

The "germ plasma," which, according to the theory of Weismann, is transmitted from cell to cell in reproduction, is assumed to be derived from the nucleus. The nucleus doubtless exercises a controlling influence not only over cell-division but also over the nutritive changes which occur in the cell protoplasm.