[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 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 laminae.  The laminae mostly show this branching as if a number of different growing points had arisen, each to take on a function similar to the epithelial process as it at first appeared.