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 ANATOMY apparently casual in its form of mesliwork, there is reason to believe that there are definite architectural laws according to which the chromatin filament is arranged, and some of these peculiarities in arrangement become obvious when a cell is about to divide (see Ency. Brit. vol. xx.. pp. 416417). In that condition the nuclear membrane disappears, and the chromatin filament forms a wreath through which a spindle of achromatin passes, at each pole of which a definite achromatic mass or centrosome accumulates, surrounded by a series of rays of achromatin. The equatorial chromatic wreath resolves itself into loops arranged witlj their closed ends directed inwards towards a central point and their free ends outwards. These loops undergo horizontal cleavage from looped to free end,, and the looped ends pass along the surface of the spindle towards pole and antipole. When thus separated,, the cell becomes mesially constricted and ultimately divides. Cells may be of very varied shapes and differ among themselves in structure and function, as noted in subsequent sections of this article; but they are, in the respects above

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elongated cell-bodies whose cytomiton is so arranged that the contraction of the protoplasm of the cell can only act in one direction, shortening the fibre along its long axis. Fibres in connective tissue (Fig. 3) are for the most part of a different kind, consisting of elongated threads of intercellular substance differentiated from the material around them by virtue of the deposit of rows of granules or linear masses of some particular protoplasm-derivate. The nature of these granules determines the kind of fibre formed, and they are in this way marked off from the formless ground-substance with which they are surrounded, being usually separate from the processes of the branched connective tissue cells. It is possible, however, that some fibres in areolar tissue may be derived from elongated cell processes which have lost their connexion with a cellbody. Histology.

Form-elements when aggregated into continuous masses make up tissues, which may be classified according to their embryological history, their position, and their ^ jttelial properties into four great classes—epithelial, t^sue. nervous, muscular, and connective. Epithelial tissue (see Ency. Brit. vol. i. p. 847 and xii. 5) is the lineal descendant of the superficial and the deepest of the three primary laminae of the embryo {lb. vol. xx. p. 418). It is usually disposed as the boundary layer of a free surface, preventing the leakage of the lymph or nutritive fluid in the underlying intercellular spaces. The amount of intercellular substance in epithelium is minimal, usually only represented by a layer of cement between contiguous cells. Epithelial tissues differ widely among themselves in properties according to the situation which they occupy j with these physiological variations are correlated morphological differences in the forms and structure of the cells. The conditions determining divergence are degree of superficial pressure, of lateral tension, the nature of the material in contact with their free surfaces, and their degree of nutrition. When stratified in comparatively thick masses, as in the epidermis, the intermediate layers consist of prickle cells (see vol. i. p. 897), whose sux faces are beset with processes which join similar processes of other cells, leaving between them lymph spaces for nutrition. In some such cases, it appears from the observation of Cleland and Thomson that the most active cell multiplication is in this layer. In general, however, Fio. 3.—Connective tissue, showing cells fibres and epithelial masses are sparingly provided with lymph X 350. (Szymnowicz.) c, cell; e, elastic fibril, /, white fib. channels, and cell growth proceeds from the deepest cells enumerated, built on a common plan. (For further parti- towards the surface. culars on cell-structure, see the text-books of histology Epithelial cells, whose surfaces are in contact with currents of above quoted, also the text-books of. embryology ot v air or of fluid, which requires to be passed on and not absorbed, Kolliker, Minot, Kollmann, and Hertwig, and the several are beset by cilia (see vol. i. p. 847). Recently it has been shown that the organization of these cells is much more complex than had numbers of Carnoy’s periodical La Cellule.). supposed (see Heidenhain, jinat. Jinzciger, 1899, No. 5), and The structureless material known as intercellular been that the cilia are connected with specialized portions of the ground-substance varies in quantity in different tissues ot reticulum of the cell substance, so that their motion is really a the body, being sometimes only a slight .cement part of the motion of the whole reticulum. Such cilia are found Inter. layer joining contiguous cells, sometimes a in the nostrils, pulmonary air-passages, Eustachian and Fallopian tubes, and a few other places. (For further particulars concerning Zt'tZce substance in which cells are embedded. substance. copious ^^ ^ either by the cilia, see Henneguy, Archives d'Anat. microscop. 1898 ; Studnicka, it hag originated Sitzb. d. kgl. bohmischen Gcsellsch. 1899; Gurwitsch, Anat. it • jexudation of material from cells or by the transformation Anzeiger, X’AA).) Nerve-endings are minute and with difficulty traceable m ordiof the periphery of a cell. Chemically, it is made up of epithelium; but in some cases there is a much more abundant some protoplasm-derivate, which varies m its nature in nary nervous supply, which reaches its climax in the organs of the different tissues. . special senses. The essential character of each of these is the Fibres are not, strictly speaking, a single category presence of one or more layers of sensory epithelium, whose comcomparable with cells. A fibre is an elongated, thread- ponent cells are elongated on their free surface into one or more like form-element, but fibres of different km s exceedingly fine rod-like processes. These and the reticulum ot Fibres. pave different morphological values. Some, the cell-body are organized in a more or less complex manner according to the nature of the impulse to which they are designed such as nerve fibres, are elongated processes of cells, made to respond by vibration, and each cell is in direct connexion wit of the same material as the cell-body, and often enclosed a nerve-ending. The forms of sensory epithelium are described in with the specific sense organs (vol. i. p. 884 et seqq.). in a tubular sheath of the nature of an investing layer of connexion aes intercellular material. Others, such as musc.e fibres, are V(For further researches, see Ramon y Cajol,S. I.Le—re tine 50