76 EPITHELIUM. 
stitute the other modification in the form of the epithelial cells. 
They were discovered by Henle in the intestinal mucus-mem- 
brane. Thev likewise enclose the characteristic nucleus, and are 
arranged with their longest sides in apposition. Their blunt 
ends are turned outwards and free. The opposite end either 
terminates abruptly also, as in the chorion, or proceeds to a 
point. This tapering figure frequently commences at the upper 
part, so that the cells then have the form of a poimted cone, 
the base of which is turned towards the outside. Henle found 
that the cilia stand upon the free surfaces of the epithelial 
cylinders in those membranes which present the phenomenon of 
ciliary motion, a fact of itself sufficient to show that the epithe- 
lium ought not to be regarded as a mere inanimate covering 
to the organized structures. 
With regard to the formation of the epithelial cells, Henle 
has already proved the rete Malpighii to consist of round 
nucleated cells, probably the young epidermal cells, and also 
that the diameter of the cells increases towards the outside, so 
that in the foetal pig he was enabled to trace the gradual transi- 
tion of the cells of the rete Malpighii into those of the epidermis. 
(Symbol ad anatomiam villor. intest., p.5.) An actual growth 
of the epithelial cells thus became very probable ; I have likewise 
followed this process in the foetal pig. The uppermost layer of 
the epidermis is there formed of large, tabular, hexagonal cells, 
furnished with a nucleus. Immediately beneath these le 
nucleated cells, which are already much smaller, and almost 
round, so that the flattening must take place very rapidly. The 
farther you proceed from the surface the smaller the cells be- 
come, and the closer they encompass the nucleus. The size 
of the nucleus also diminishes in some degree, but by no means 
in the same proportion. In the lowest strata, the cells cannot 
any longer be distinguished, but the nuclei le close together, 
with a small quantity of minutely granulous intermediate sub- 
stance. It is, however, very difficult to obtain positive convic- 
tion of this fact, for the stratum of nuclei is too firmly connected 
with the cutis. We shall have an opportunity of observing 
this relation of the nuclei more distinctly hereafter in the 
feather. The mode of formation is probably this : cell-nuclei are 
formed, in the first place, immediately upon the surface of the 
cutis; and then around, and closely encompassing them, the cells. 
