A.NIMAL.] HISTOLOGY cells which either still have their place in the general layer of the ectoderm or are but imperfectly separated from it ; and here, again, the commencing separation does not occur at one part only, but over extensive tracts of the surface. Nevertheless these cells are modified in structure precisely in the same way as those which in higher animals are derived from the mesoderm. The nervous functions are also performed by cells and fibres, which, although they show those modifications of structure which iu the higher animals are characteristic of nervous tissue, yet remain strictly confined to the ectoderm, and do not, as in the higher animals, penetrate into the mesoderm. The Epithelial Tissues. Although, as we have seen (see p. 4, note 2), the layers of cells which are first formed are layers of epithelium, and, therefore, the epithelial tissues are the first to be produced, nevertheless we find that they undergo less modification in structure than any of the other three classes of tissue. As before said, they invariably con sist merely of cells cemented together by an imperceptible amount of intercellular substance, 1 and the cells them selves only show minor degrees of modification in shape and structure, at least as compared with the other tissues constituted mainly of cells, namely, the muscular and nervous. Modifications in Shape of Epithelial Cells. The cells of this tissue may be either elongated and set like palisades over the surface which they cover, in which case they are termed " columnar " (fig. 3), or they may be flattened out over the surface, and they then appear as thin "scales"; and every variation in shape is met with between these two extremes. In any case where they form a single layer, since the cells are set closely together, the mutual apposition of neighbouring cells produces a flattening of the opposed sides, so that, when the epithelium is looked at from the surface, the cells have a polygonal outline, and collectively present the appearance of a mosaic pavement (fig. 4). In 3. Fig. 4. Pig. 5. FIG. 8. Columnar epithelium cells. FIG. 4. Mosaic appearance of epithelium. FIG. 5. Stratified epithelium. certain cases, especially where there is liability to abrasion of the surface which they cover, the epithelial cells are dis posed in two, three, or more superimposed layers (fig. 5), and then the cells of the different layers may vary much in size, shape, and consistence. Such an epithelium is termed " stratified." It frequently happens that the layer of epithelium which covers a surface is prolonged into depressions, which may be quite simple or may be ramified either slightly or in the most complex manner. The epithelial cells which lino such depressions may resemble those of the surface upon which the depression opens, or they may become more or less modified in size, shape, and other particulars, and con stitute themselves into a distinct variety of epithelial tissue. Since depressions like those just mentioned are generally for the purpose of forming some special secretion, and are termed fjlands, and since this secretion is elaborated by the agency of the epithelial cells which line the gland, any such special 1 Exceptions to this general rule are seen in the layer of cells which underlies the chitinous cuticula of annelids (Claparede), and in the enamel organ of the developing teeth of vertebrates (Kolliker), where the cells, although epithelial, are ramified and united by their branches into a network; and, in a far less degree, in many of the lower cells of a stratified epithelium, where it can be seen with a high power that the edges and surfaces of the cells are provided with numerous short projections which are applied to those on adjacent cells. variety of epithelium is termed a "glandular" or "secreting" epithelium. Modifications in Structure. The modifications in struc ture which the cells of epithelial tissue undergo are com paratively slight. One of the most common is the conver sion of the external layer of the protoplasm of the cell into a firm membrane, generally of a horny nature, but this membrane is seldom sharply marked off from the substance of the cell, as is the case with the cellulose membrane of the vegetable cell. It becomes formed, moreover, to a very different extent in different cells, according to the function which the particular epithelium has to perform; where, for example, the epithelium is almost purely a protective cover ing, as in the stratified scaly epithelia, a considerable part, or even the whole thickness of many of the epithelial cells, is thus transformed ; but where, on the other hand, the cells have to play an active part in yielding a secretion to moisten the surface, or in protruding a portion of their protoplasm in the form of vibratile cilia to produce currents over the surface, or to move the organism through the water, we find little, if any, of such conversion of the superficial cell-substance. What little there may be is con fined to the attached surfaces of the cell, or if there is any such covering on the free surface, it is penetrated by porea which allow of a communication between the protoplasm of the cell and the external medium. Another common modification of structure which epi thelial cells exhibit is the existence of vibratile cilia at the free surface (fig. 6). This, again, is especially frequent with cells of a columnar shape, but it may occur in any. The cilia appear to be protrusions of the more active external protoplasm of the cell, which are in most cases incapable of being again withdrawn, and are in all probability modified in minute structure, although they are always so small that such modi fication, if it exists, escapes detection even with the employment of the highest powers of the microscope. At their base, however, the cilia arc certainly con tinuous with the unaltered proto plasm of the cell. This may be Fig. 7. Fig. 8. FIG. C. A ciliated epithelium cell. FIG. 7. A striated epithelium cell. FIG. 8. A ciliated cell with one large ciliura. Fia. 9. Sense-epithelium cell of Awelia. seen even where the cilia are small and spring in a bunch from the free surface of the cell, but much better in those kinds of ciliated epithelium in which but a single large cilium is connected to each columnar cell (fig. 8). Many epithelial cells, especially those of secreting glands, show a differentiation of their protoplasm in the form of fine striae or rods which pass from the attached border of the cell towards the free end (fig. 7). Cells thus modified are found in the ducts of the salivary glands, in the alveoli of the pancreas, and in the convoluted tubules of the kidney in Vertebrata. One of the most remarkable modifications which epithe lium cells exhibit is found in the organs of special sense. This is the presence of a fine filamentous process or processes springing from the free surface of the epithelium cells, and resembling in their appearance long cilia, but not spontane
ously vibratile (fig. 9). Moreover, the cells in question,
