It results from all this, that, among the higher animals, the true homologue of the integu ment of the Hydra is the epidermic layer alone. But it would be exceedinglyinconvenient to change the accepted meaning of " Integu ment " on this ground ; and, therefore, I shall, throughout the present article, consider as integument — the outermost plane of indif ferent tissue in the animal body, with its external and internal arece of metamorphosis collectively ; these being simply the expressions of two pro cesses of growth in opposite directions, and their line of contact.
lt must not be supposed that this phrase ology involves any hypothetical views: the fact that any integumentary organ consists of these three portions will be found to be either distinctly stated or implied by all writers, and is indeed obvious enough on inspection. But though the facts be old enough, this ex pression of them is unfortunately so new, that I know of no existing terminology by which it can be properly enunciated. The term "Epidermis," for instance, at present, though it denotes the important character of the direction of growth to which I refer, implies even more strongly the simple cellular struc ture of an organ; so that to speak of Epi dermic" bony or fibrous tissue would sound almost contradictory. Again, all these distinc tions, which have been shown to exist between the tw o elements of the integument, equally hold good with regard to the mucous mem branes. Now we have a term "Epithelium" for the epidermic element of the latter ; but there is, as far as I know, none for the ele ment which corresponds w ith the derma. Nor have we any word for the boundary line be tween the endogenous and exogenous arem of growth— the term " basement membrane" expressing only an accidental character of the tissue immediately on one or the other sides of that line.
Although with great reluctance, then, I feel compelled to propose two or three new terms, which may have general application, not only to the integumentary organs, but to all other membranes which possess free surfaces and definite directions of growth and meta morphosis.
The boundary line — passing through in different tissue—between any two such op posite arem of growth and metamorphosis, I term the Protonwrphic line. The whole ex ternal (free) area of metamorphosis I call the Ecderon ; the entire internal (deep) area of metamorphosis, the Enderon.
It will be observed that these definitions rest wholly upon the mode of growth, and leave altogether out of consideration the structure of' the resulting tissue. In fact, as I have al ready said, an extensive study of the integu mentary organs convinces one at once that mere structure affords no base for homology ; the ecderon, for instance, presenting every variety from the structurelessness of a homo geneous membrane, as in the Tmniadm, to the complex combination of the so-called enamel, dentine and bone, in the scales of Placoid Fishes.
It is, I venture to think, no small evidence in favour of the importance of these consi derations that they enable us to carry still further the doctrine of the identity of struc ture of plants and animals sketched by Cas par Wolff, and developed in our own times by Schwann. If we make a transverse sec tion of the growing limb of a vertebrate ani mal, leaving out of consideration, for the moment, the vessels, nerves, and muscles, we observe from the surface inwards, 1st, the ecderonic area of metamorphosis ; 2nd, the integumentary protomorphic line ; 3rd, the enderonic area of metamorphosis ; 4th, the periosteal area of metamorphosis ; 5th, the prototnorphic line, formed by the indifferent tissue between periosteum and bone ; 6th, the osteal area of metamorphosis, within which lies, 7th, the cartilage resulting from the me tamorphosis of the tissue of the primitive axis of the limb.
Now, if we compare this with the growing shoot of a young exogenous plant, we meet with exactly the same series from without inwards. There is, 1st, the epidermis, which commonly becomes replaced by a cork or peri dermal layer, just as the primary epidermis over a nail is thrust aside by the subjacent and subsequently-formed horny matter ; or, as the horny "epidermis" of a Skate is pushed aside and replaced by the calcareous placoid spine. Beneath this lies, 2nd, a prototnorphic (or cambial) line, from which metamorphosis into periderma goes on outwards, while inwards it passes into, 3rd, the metamorphosed tissue of the niesophlceum. Next to this comes, 4th, the metamorphic area of the endophlceum or liber ; within which is, 5th, the proto morphic line of the cambium, which becomes metamorphosed on its inner surface into, 6th, the wood ; within which lies, 7th, the pith, the result of the metamorphosis of the pri mitive axis of the shoot.