distinguished as has just been said from the cell-substance in which it lies. Very frequently, but by no means always, the outer layer of the cell-substance is hardened into a distinct casing or envelope, the cell-wall, the cell then becoming an undeniable vesicle, and the cell-substance being often spoken of as the cell contents. The cellsubstance may remain as soft semi-solid protoplasm, or may be hardened in various ways, or may be wholly or partially liquefied; in the latter case a cell-wall is naturally always present.

A, vertical section of a layer of epidermis, or epithelium, from its free to its deep surface. B, lateral views of the cells of which this layer is composed at different heights: a, cell in the deepest layer, and therefore most recently formed and least altered; b, cell higher up, and therefore somewhat changed; c, d, cells still more changed, and much flattened. C, scales such as d viewed from their flat sides. (Magnified about 250 diameters.)

As development goes on, the nuclei simply increase in number by division and subdivision, without undergoing any marked change;1 but the substance in which they are

• Each nucleus divides into two, and each half soon grows up into the size of the parent nucleus. While this is going on the matrix round the nuclei also divides, each new nucleus having a quantity of matrix allotted to it, so as to form a new cell exactly like the old one, from which it sprang.

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imbedded, becomes very variously modified, both chemically and structurally, and gives rise to those peculiarities by which completely formed tissues are distinguished from one another.

3. In the adult body the simplest forms of tissue, i.e. those in which the matrix has been least changed, are perhaps the various kinds of epithelium (including the epidermis).

These are distinctly cellular in nature, that is, the portion of the matrix belonging to each nucleus can, with a little pains, be recognized as distinct from the portions belonging to the other nuclei. In fact they differ from white blood corpuscles chiefly in two points: firstly, the matrix of each cell becomes more or less chemically changed so as to lose its soft protoplasmic nature (and at the same time its power of executing amoeboid movements); and, secondly, takes on a rigid definite form, which may or may not be globular. These epithelial tissues are constantly growing in their deepest parts, and are, as constantly, being shed at their surfaces.

The deep part consists of a layer of such globular, nucleated cells as have been mentioned, the number of which is constantly increasing by the spontaneous division of the nuclei and cells. The increase in number thus effected causes a thrusting of the excess of cell population towards the surface; on their way to which they become flattened, and their walls acquire a horny texture. Arrived at the surface, they are mere dead horny scales, and are thrown off (Fig. 86).

Epithelium of the kind just described is called squamous. It is found in the mouth, and its scales may always be obtained in abundance by scraping the inside of the lip.

Epidermis consists of exactly similar cells, except that the conversion of the topmost cells into horny scales is still more complete. The nucleus, too, is eventually lost. The deep layers of epidermis, consisting of softer cells not yet flattened or made horny, often form quite a distinct part, and these are often spoken of as the rete mucosum. (See Fig. 32, b; Fig. 88, C, d.)

In other parts of the alimentary tract, as in the intestines, the full-grown epithelial cells are placed side by side with one another, and perpendicular to the surface of

the membrane.

Such epithelium is called cylindrical

(Fig. 46, b, b'), or columnar.

In some places, such as in the gastric glands, in some parts of the kidney, in the ureters and elsewhere, the epithelial cells remain globular or spheroidal.

Squamous epithelium generally consists of many layers of cells, one over the other; in other forms of epithelium there are few, in some cases apparently only two layers.

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the submucous vascular tissue; b, the deep layer of young epithelium
cells; c, the cylindrical full-grown cells, with (d) the cilia. (Magnified
about 350 diameters.)

Ciliated epithelium is usually of the cylindrical kind, and differs from other epithelium only in the circumstance that one or more incessantly vibrating filaments are developed from the free surface of each cell. (Sce Lesson VII. § 3.)

4. In certain regions of the integument, the epidermis becomes metamorphosed into nails and hairs.

Underneath each nail the deep or dermic layer of the integument is peculiarly modified to form the bed of the nail. It is very vascular, and raised up into numerous parallel ridges, like elongated papillæ (Fig. 88, B, C). The surfaces of all these are covered with growing epidermic cells, which, as they flatten and become converted into horn, coalesce into a solid continuous plate, the nail. At the hinder part of the bed of the nail, the integument forms a deep fold, from the bottom of which, in like manner,

new epidermic cells are added to the base of the nail, which is thus constrained to move forward.

A, a longitudinal and vertical section of a nail: a, the fold at the base of the nail; 6, the nail; c, the bed of the nail. The figure B is a transverse section of the same-a, a small lateral fold of the integument; b, nail; c, bed of the nail, with its ridges. The figure C is a highly-magnified view of a part of the foregoing-c, the ridges; d, the deep layers of epidermis; e, the horny scales coalesced into nail substance. (Figs. A and B magnified about 4 diameters; Fig. C magnified about 200 diameters.)

beyond the end of the finger, where it is worn away, or cut off.

a, shaft of hair above the skin; b, cortical substance of the shaft, the medulla not being visible; c, newest portion of hair growing on the papilla (i); d, cuticle of hair; é, cavity of hair sac; f, epidermis (and root-sheaths) of the hair-sac corresponding to that of the integument (m); g, division between dermis and epidermis; h, dermis of hair-sac corresponding to dermis of integument (); k, mouths of sebaceous glands; n, horny epidermis of integument.