The main portion of the wall is developed from the numerous papillae covering the corium of the coronary cushion.  We have in this way numberless down-growing tubes of horn.  Professor Mettam describes their formation in a singularly happy fashion:  “Let the human fingers represent the coronary papillae, the tips of the fingers the summits of the papillae, and the folds of skin passing from finger to finger in the metacarpo-phalangeal region the depressions between the papillae.  Imagine that all have a continuous covering of a proliferating epithelium.  Then we shall have a more or less continuous column of cells growing from the tip of the finger or papilla (a hollow tube of cells gradually moving from off the surface of the finger or papilla like a cast), and similar casts are passing from off all the fingers or papillae.”

From this description it will be noticed that each down-growing tube of horn bears a striking resemblance to the growth of a hair, described on p. 47.  In fact, the horn tube may be regarded as what it really is, a modified hair.

We next continue Professor Mettam’s illustration, and note how the modified hairs or horn tubes become as it were matted together to form the hoof wall.  The cells lining the depressions are also proliferating, and their progeny serve to cement together the hollow casts of the papillae, thus giving the inter-tubular substance.  We have thus produced hollow tubes, united together by cells, all arising from the rete Malpighii of the coronary corium.  Section of the lower part of the horn tubes shows them to contain a cellular debris.

Thus, in all, in the horn of the wall we find a tubular, an intertubular, and intratubular substance.  In fact, hairs matted together by intertubular material, and only differing from ordinary hairs in their development in that they arise, not from papillae sunk in the corium, but from papillae projecting from its surface.

Although this disposes of the wall proper, there still confronts us the question of the development of the horny laminae.  To accurately determine this point it is absolutely essential to examine, histologically, the feet from embryos.

In the foot of any young ungulate in the early stages of intra-uterine life horizontal sections will show a covering of epidermis of varying thickness.[A] This may be only two or three cells thick, or may consist of several layers.  Lowermost we find the cells of the rete Malpighii.  As some criterion of the activity with which these are acting, it may be noted that with the ordinary stains their nuclei take the dye intensely.  The cells of this layer rest upon a basement membrane separating the epidermis from the corium.  At this stage the corium has a perfectly plane surface.

[Footnote A:  Equine foetus, seventy-seven days old.]

[Illustration:  FIG. 26.—­SECTION OF FOOT OF EQUINE FOETUS, SEVENTY-SEVEN DAYS OLD.  The rete Malpighii rests on a plane corium; the rent in the section is along the line of the cells of the rete (Mettam).]