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 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 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).]