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 papillæ, 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 papillæ sunk in the corium, but from papillæ
projecting from its surface.

Although this disposes of the wall proper, there still confronts us the
question of the development of the horny laminæ. 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