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).]

[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 laminæ.[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 échelon_ into its connective
tissue. Each point of the ingrowing lines of the _échelon_ 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_
laminæ, and cut up the corium into _sensitive_ laminæ) are free from
irregularities or secondary laminæ. Before these are to be observed other