eated layer out of which the
epidermis grows, or where this layer has been destroyed by an extensive
burn, the process of healing is very significant. From the subjacent
tissues, which in the normal order have no concern with outward growth,
there is produced a new skin, or rather a pro-skin; for this substituted
outward-growing layer contains no hair-follicles or other specialities
of the original one. Nevertheless, it is like the original one in so far
that it is a continually renewed protective covering. Doubtless it may
be contended that this make-shift skin results from the inherited
proclivity of the type--the tendency to complete afresh the structure
of the species when injured. We cannot, however, ignore the immediate
influence of the medium, on recalling the facts above named, or on
remembering the further fact that an inflamed surface of skin, when not
sheltered from the air, will throw out a film of coagulable lymph. But
that the direct action of the medium is a chief factor we are clearly
shown by another case. Accident or disease occasionally causes permanent
eversion, or protrusion, of mucous membrane. After a period of
irritability, great at first but decreasing as the change advances, this
membrane assumes the general character of ordinary skin. Nor is this
all: its microscopic structure changes. Where it is a mucous membrane of
the kind covered by cylinder-epithelium, the cylinders gradually
shorten, becoming finally flat, and there results a squamous epithelium:
there is a near approach in minute composition to epidermis. Here a
tendency towards completion of the type cannot be alleged; for there is,
contrariwise, divergence from the type. The effect of the medium is so
great that, in a short time, it overcomes the inherited proclivity and
produces a structure of opposite kind to the normal one.

With but little break we come here upon a significant analogy, parallel
to an analogy already described. As was pointed out, an inorganic body
that is modifiable by its medium, acquires, after a time, an outer coat
which has already undergone such change as surrounding agencies can
effect; has a contained mass which is as yet unchanged, because
unreached; and has a surface between the two where change is going on--a
region of activity. And we saw that alike in the vegetal cell and the
animal cell there exist analogous distributions: of course with the
difference that the innermost part is not inert. Now we have