EPITHELIUM
stitute the other modification in the form of the epithelial cells. They were discovered by Henle in the intestinal mucus-membrane. They likewise enclose the characteristic nucleus, and are arranged with their longest sides in apposition. Their blunt ends are turned outwards and free. The opposite end either terminates abruptly also, as in the chorion, or proceeds to a point. This tapering figure frequently commences at the upper part, so that the cells then have the form of a pointed cone, the base of which is turned towards the outside. Henle found that the cilia stand upon the free surfaces of the epithelial cylinders in those membranes which present the phenomenon of ciliary motion, a fact of itself sufficient to show that the epithelium ought not to be regarded as a mere inanimate covering to the organized structures.
With regard to the formation of the epithelial cells, Henle has already proved the rete Malpighii to consist of round nucleated cells, probably the young epidermal cells, and also that the diameter of the cells increases towards the outside, so that in the foetal pig he was enabled to trace the gradual transition of the cells of the rete Malpighii into those of the epidermis. (Symbolae ad anatomiam villor. intest., p.5.) An actual growth of the epithelial cells thus became very probable; I have likewise followed this process in the foetal pig. The uppermost layer of the epidermis is there formed of large, tabular, hexagonal cells, furnished with a nucleus. Immediately beneath these lie nucleated cells, which are already much smaller, and almost round, so that the flattening must take place very rapidly. The farther you proceed from the surface the smaller the cells become, and the closer they encompass the nucleus. The size of the nucleus also diminishes in some degree, but by no means in the same proportion. In the lowest strata, the cells cannot any longer be distinguished, but the nuclei le close together, with a small quantity of minutely granulous intermediate substance. It is, however, very difficult to obtain positive conviction of this fact, for the stratum of nuclei is too firmly connected with the cutis. We shall have an opportunity of observing this relation of the nuclei more distinctly hereafter in the feather. The mode of formation is probably this: cell-nuclei are formed, in the first place, immediately upon the surface of the cutis; and then around, and closely encompassing them, the cells.
