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 by Treviranus. Not only was attention thus gradually directed to the importance of the cell-contents, but observations were not lacking, even in the plant kingdom, tending to weaken the importance hitherto attached to the cell-wall. Among these may be mentioned Cohn’s observation that in the reproduction of Algal forms the protoplasm contracts away from the cell-wall and escapes as a naked “swarm spore.” Similarly in the animal kingdom instances began to be noted in which no membrane appeared to be present (Kolliker, 1845; Bischoff, 1842), and for some time it was hotly debated whether these structures could be regarded as true cells. As a result of the resemblance between the streaming movements in these apparently naked cells (e.g. lymphocytes) and those seen in plant cells, R. Remak was led (1852–1853) to apply Mohl’s term “protoplasm” to the substance of these animal cells also. Similarly Max Schultze (1863) and H. A. de Bary (1859), as a result of the study of unicellular animals, came to the conclusion that the substance of these organisms, originally termed “Sarcode” by F. Dujardin, was identical with that of the plant and animal cell. Numerous workers now began to realize the subordinate position of the cell-wall (e.g. Nägeli, Alexander Braun, Leydig, Kolliker, Cohn, de Bary, &c.), but it is to Max Schultze above all that the credit is due for having laid the foundation of the modern conception of the cell—a conception often referred to as the proto-plasmic-theory in opposition to the cell-theory of Schleiden and Schwann. Max Schultze showed that one and the same substance, protoplasm, occurred in unicellular forms and in the higher plants and animals; that in plants this substance, though usually enclosed within a cell membrane, was sometimes naked (e.g. swarm spores), while in many animal tissues and in many of the unicellular forms the cell-membrane was always absent. He therefore concluded that in all cases the cell-membrane was unessential, and he redefined the “cell” of Schleiden and Schwann as “a small mass of protoplasm endowed with the attributes of life” (1861). In the same year the physiologist Brücke maintained that the complexity of vital phenomena necessitated the assumption for the cell-protoplasm itself of a complex structure, only invisible because of the limitations of our methods of observation. The cell in fact was to be regarded as being itself an “elementary organism.” By this time too it was realized that the formation of cells de novo, postulated by Schleiden’s theory of “phytogenesis,” did not occur. Cells only arose by the division of pre-existing cells,—as Virchow neatly expressed it in his since famous aphorism, omnis cellula e cellula. It was, however, many years before the details of this “cell-division” were laid bare (see Cell-Division below).

General Morphology of the Cell.—In its simplest form the cell is a more or less spherical mass of viscid, translucent and granular protoplasm. In addition to the living protoplasm there is present in the cell food-material in various stages of assimilation, which usually presents the appearance of fine granules or spherules suspended in the more or less alveolar or reticular mesh-work of the living protoplasm. In addition there may be more or less obvious accumulations of waste material, pigment, oil drops, &c.—products of the cell’s metabolic activity. All these relatively passive inclusions are distinguished from the living protoplasm by the term “metaplasm” (Hanstein), or “paraplasm” (Kupffer), although in practice no very sharp distinction can be drawn between them. The cell is frequently, but by no means always, bounded by a cell-wall of greater or less thickness. In plants this cell-wall consists of cellulose, a substance closely allied to starch; in animals only very rarely is this the case. Usually the cell-wall, when this is present, is a product of the cell’s secretive activity; sometimes, however, it appears to be formed by an actual conversion of the surface layer of the protoplasm, and retains the power of growth by “intussusception” like the rest of the protoplasm. Even when a limiting membrane is present, however, evidence is steadily accumulating to show that the cell is not an isolated physiological unit, but that, in the vast majority of cases, there is a protoplasmic continuity between the cells of the organism. This continuity, which is effected by fine protoplasmic threads (“cell-bridges”) piercing the cell-wall and bridging the intercellular spaces when these are present, is to be regarded as the morphological expression of the physiological interdependence of the various—often widely separated—tissues of the body. It is probable that it is the specialization of this primitive condition which has produced the cell-elements of the nervous system. In many cases the cell-connexions are so extensive as to obliterate cell-boundaries. A good example of such a “syncytial” tissue is provided by the heart muscle of Vertebrates and the intestinal musculature of Insects (Webber).

In all multicellular, and in the great majority of unicellular, organisms the protoplasm of the cell-unit is differentiated into two very distinct regions,—a more or less central region, the nucleus, and a peripheral region (usually much more extensive), the cell-body or cytoplasm. This universal morphological differentiation of the cell-protoplasm is accompanied by corresponding chemical differences, and is the expression of a physiological division of labour of fundamental importance. In some of the simpler unicellular organisms, e.g. Tetramitus, the differentiated protoplasm is not segregated. Such forms are said to have a “distributed” nucleus, and among the Protozoa correspond to Haeckel’s “Protista.” It is probable that among plants the Bacteria and Cyanophyceae have a similar distributed nucleus. In all the higher forms, however, the segregation is well marked, and a “nuclear membrane” separates the substance of the nucleus, or “karyoplasm” from the surrounding “cytoplasm.” Within the nuclear membrane the karyoplasm is differentiated into two very distinct portions, a clear fluid portion, the “karyolymph,” and a firmer portion in the form of a coarser or finer “nuclear reticulum.” This latter is again composed of two parts, the “linin reticulum,” and, embedded in the latter and often irregularly aggregated at its nodal points, a granular substance, the “chromatin,” the latter being the essential constituent of the nucleus. In addition to the chromatin there may be present in the nucleus one or more, usually spherical, and as yet somewhat enigmatical bodies, the “nucleoli.” In addition to the nucleus and cytoplasm, a third body, the “centrosome,” has often been considered as a constant cell-structure. It is a minute granule, usually lying in the cytoplasm not far from the nucleus, and plays an important part in cell-division and fertilization (see below).

Cell-differentiation.—Both among unicellular and multicellular individuals the cell assumes the most varied forms and performs the most diverse functions. In all cases, however, whether we examine the free-living shapeless and slowly creeping Amoeba, or the striped muscle cell or spermatozoon of the Metazoa (fig. 1, b and c), the constant recurrence of cytoplasm and nucleus show that we have to deal in each case with a cell. The variation in the form and structure of the cell is an expression of that universal economic law of nature, “division of labour,” with its almost invariable accompanying “morphological differentiation”; the earliest and most fundamental example being in the differentiation of the cell-protoplasm into cytoplasm and nucleus. In multicellular individuals the division of labour to which the structural complexity of the organism is due is between the individual cell-units, some cells developing one