1. Newly formed cell structure; 2. Division of the nucleus; 3. It changes its situation in the cell; 4. Subdivides and breaks up; 5. Cell-walls increase in thickness; 6. Branch out into stellate cells; 7. Two cells coalesce; 8 and 9. Become multicellular.
Epithelial Cells.—All free surfaces of the human body, both internal and external, are to a very considerable extent covered by epithelium cells. These cells are everywhere the same, but with modifications in shape and arrangement. Epithelial cells are nucleated and always joined by their surfaces or edges, without, on the external surfaces, the intervention of connective tissue.
There are four essential varieties:—1. Tesselated; 2. Columnar; 3. Spheroidal; 4. Ciliated; in all of which the nucleus remains remarkably uniform in its characters, is either round or oval, and flattened out, measuring 1⁄6000th to 1⁄4000th of an inch in diameter. They are insoluble in acetic acid, colourless, or slightly tinted by the structure with which they are in contact, and usually contain one or more nucleoli with a few minute irregular granules, as represented in [Fig. 424].
The simplest and most commonly distributed variety is the tesselated, known also as the scaly, squamous, pavement, and flattened epithelium, always arranged in single layers, lining serous cavities, many parts of the mucous membrane, and the interior of ducts and blood vessels. Upon the external surface of the body it occurs in superimposed layers, forming the “stratified epidermis.” To obtain specimens of lamellar epithelium it is only necessary to collect a little saliva, or pass a glass slide over the lining membrane of the cheek, cover it with a thin cover glass, and examine it with a ¼-inch objective. Pavement epithelium is the elementary structure of hair, nails, and horn.
PLATE XIX.
ANIMAL TISSUES.
Columnar epithelium exists upon the mucous membrane of the stomach, on the villi of the intestines, and in the several canals. It occupies either a vertical or horizontal position, and may be detached in rows, as shown in [Plate XIX]., No. 2, a section taken from the intestine of a rabbit. This variety, when more highly magnified, as in [Fig. 424], is seen to consist of club-shaped nucleated cells, the thicker end being turned towards the surface. The protoplasm of the cell is granular, and the presence of minute vacuoles and fatty globules occupy a great part of the space. The nucleus is now seen to contain a fine network. At times the outer end of the cell is distended, as in [Fig. 3]. This form of columnar epithelium (known as the “goblet” cell) presents a close and remarkable resemblance to the cilio-flagellate “collared” infusorial monad in its extended “wine-glass” form.
Fig. 424.