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82 FEATHERS. guish the particular wall of every cell. But when the foetus has lain for a time in strong spirit, the horny substance of the hoof may be easily separated from the foot, in consequence of the connexion between the cells having become looser. The undermost layers of cells, however, remain attached to the foot. The interior of the layer of horny substance so separated, consists of a crumbling mass, somewhat resembling a boiled yelk. The particles cannot, however, be separated quite so readily from one another as those of the yelk are. With the aid of the microscope, this mass is found to be composed of irregularly angular bodies, resembling the yelk-substance when boiled. These bodies are the isolated cells, whose peculiar walls are distinctly perceptible, and some few of them have a nucleus, which lies upon the inner surface of their wall. A continuous firm layer of flat epithelium-scales, the immediate continuation of the outer lamellae of the epidermis, consisting of flat cells, surrounds these polyhedral cells as an external covering to the entire hoof. This lamella exists in the foetal pig at a very early age, the layer of polyhedral cells being at that time very slight; in a more advanced stage of development, however, the latter forms the chief mass of the horny substance of the hoof. In the recent condition these cells must also have somewhat firm contents, otherwise, with so delicate a cell-membrane, the substance could not be so firm. But its elasticity was such as to prevent my crushing one of the cells with the compressorium, my object being to ob- serve whether the cell-contents would flow out, or be torn like a firm substance. As the cell-contents form a large portion of this horny substance, whilst the nails consist for the most part of flat cells without any discernible contents, almost entirely therefore of cell-walls, a chemical distinction may be presumed to exist between the two structures.

5. Feathers. The feather is composed of the quill, the shaft, and the vane, or beard. The elementary structure of these parts is, however, what most interests us at present; and in order to investigate it, at least in order to become acquainted with the relation which the different elementary formations in the feather bear to cells, we must take one in which a part of the shaft is in progress of formation. The feathers at that