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-SEV