EN
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).]

[Illustration: FIG. 27.--SECTION FROM FOOT OF SHEEP EMBRYO. It shows a
pronounced epithelial ingrowth into the corium (Mettam).]

The next stage will demonstrate the first step in the formation of the
sensitive laminae.[A] The plain surface of the corium has now become broken
up, and what is noticed is that the broken-up appearance is due to the
epithelial cells irrupting and advancing _en echelon_ into its connective
tissue. Each point of the ingrowing lines of the _echelon_ has usually
one cell further advanced into the corium than its neighbours, and may be
termed the _apical cell_. The fine basement membrane separating epithelium
from corium is still clearly evident. This epidermal irruption of the
corium takes place at definite points right round the foot. It is extremely
probable, however, that it commences first at the toe and spreads
laterally.

[Footnote A: Sheep embryo, exact age unknown.]

As yet, these cellular ingrowths (which are destined to be the _horny_
laminae, and cut up the corium into _sensitive_ laminae) are free from
irregularities or secondary laminae. Before these are to be observed other
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 laminae' (Mettam).]

With the appearance of horn comes difficulty of sectioning. The last