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.

In the centre of the processes a few nuclei may be observed, but they are
scarce, and stain only faintly; they have arisen from the cells of the rete
Malpighii which have grown into the corium. In fact, the active cells are
passing their daughters into the middle of the process, and these pass
through similar stages as those derived from the ensheathing epidermis. In
other words, the daughter cells of the constituents of the rete Malpighii
which have grown into the corium pass through a degeneration precisely
similar to that undergone by cells shed at desquamation, or those which
eventually give rise by their agglutination to a hair.

This is the real origin of the horny laminae, and the thickness of these is
increased merely by an increase in the area covered by the cells of the
rete Malpighii--i.e., by the development of secondary laminar ridges. If
a section from a foal at term be examined, the processes will be found far
advanced into the corium, and, occupying the axis of each process, will
be seen a horny plate, continuous with the horn of the wall. No line of
demarcation can be observed between the horn so formed and the intertubular
material of the wall. They merge into and blend with each other, with no
indication of their different origins. The cells that have invaded the
corium have thus _not lost their horn-forming function_. There has merely
been an increase in the area for horn-producing cells. The horny processes
are continuous with the hoof proper at the point where the epithelial
ingrowth first commenced to invade the corium, and fuses here with the
horn derived from the cells of the rete Malpighii which have _not_ grown
inwards, and which are found between the processes in the intact foot. From
this it is clear that some considerable portion of the horn of the wall is
derived from the cells of th