changes in connection with the ingrowths are to be noticed.

[Illustration: FIG. 28.--SECTION FROM CALF EMBRYO. The epithelial ingrowths
hang down from the epidermis into the corium like the teeth of a comb
(Mettam).]

The first is merely that of elongation of the epithelial processes into the
connective tissue, until the rete Malpighii gives one the impression that
it has hanging to its underneath surface and into the corium a number of
thorn-like processes. These extend all round the front of the foot, and
even in great part behind. Accompanying this elongation of the processes
is a condensation of the epithelial cells immediately above the rete
Malpighii, with a partial or total loss of their nuclei. This is the first
appearance of true horn, and its commencement is almost coincident with the
first stages of ossification of the os pedis.

[Illustration: FIG. 29.--SECTION OF AN EPITHELIAL INGROWTH FROM AN EQUINE
FOETUS. It shows commencing secondary laminar ridges. In the centre are
epithelial cells which are undergoing change into horny elements to form
the horn core, or 'horny laminæ' (Mettam).]

With the appearance of horn comes difficulty of sectioning. The last
specimen that Professor Mettam was able to satisfactorily cut upon the
microtome was from a foetus between three and four months old. In this the
secondary laminar ridges were clearly indicated, and the active layer of
the rete Malpighii could be traced without a break from one ingrowing
epithelial process to the next, and around this, following all the
irregularities of its outline, and covering the branches of the nascent
laminæ. The laminæ mostly show this branching as if a number of different
growing points had arisen, each to take on a function similar to the