A full description of the various forms of feathers is given in the article AVES in a former portion of this work, to which the reader is referred.
There can be no question as to the relations of the integumentary organs hitherto described to the primary constituents of the integuments, but it is different with regard to those cal cified tegumentary appendages, the scales of Fishes, and the so called "dermal" calcified plates of Reptilia and Mammalia. One point is quite certain with regard to these append ages, that they are not, like the calcified shells of the mollusca, the representatives of the outer portion of the originally cellular epidermis (are not therefore comparable to the " cuticula" of a plant), inasmuch as the latter may always, in their young state, be traced over them. It is for this reason, I imagine, that they are at present ordinarily called " dermal" organs. A truly dermal or enderonic organ, however, ought, if it continues to grow, to re tain the same characters as the enderon of which it forms a part. It ought, therefore, to have its protomorphic surface external and to grow exogenously. Now, no scale or plate of any fish, so far as I am aware, does this; on the other hand, it holds good of all, whether Placoid, Ganoid, Cycloid or Ctenoid#, that they cominence by the occurrence of a calcfiic deposit immediately beneath the cellular ecderon, and that they increase by continual addition to the inner surface of this primary deposit. There are two ways in which we may conceive that these scales and plates are produced. Either they are a gradual calcifi cation of the whole enderon from without inwards (which is the view taken by Leydig, of the scales of Polypterus), in which case the only tissue of the enderon capable of increase (that of the protomorphic line) being arrested by the calcareous deposit, the whole enderon at these parts must cease to grow, which would appear to be contrary to fact ; or the scale corresponds with the cork-layer of the vegetable integument, and like it, though developed beneath the ordinary cellular epi dermis, is still a truly ecderonic structure.
A great deal might be said for both these views; and if in this place, I assume the latter to be more correct, it is because I think we must be guided by the homology of the scales with certain other organs, where these rela tions are more definitely expressed. It may be taken as certain, I think, that the scales, plates, and spines of all fishes are homologous organs ; nor as less so that the tegumentary spines of the Plagiostomes are homologous with their teeth, and thence with the teeth of all verte brata. Ag,ain, it appears to me indubitable that the teeth and the hairs are homologous organs ; they are therefore either both en deronic or both ecderonic. Taking for granted the validity of a basement membrane as a mark of the boundary between ecderon and enderon, I elsewhere* arrived at the conclu sion that the teeth are enderonic organs, and that therefore the hairs must follow them. Now, however, that a " basement membrane " turns out to be no test at all, there seems no reason why we should not be guided entirely by the direction of growth, and consider both hairs and teeth as ecderonic organs ; the former being a development of the cellular ecderon, and corresponding with the ordinary horny epidermis ; the latter, a development of a deep layer of the ecderon beneath this. It appears to me that we can do no other than admit this view for the teeth ; but if this be the case, we ;nay apply it to the scales of fish (and the " dermal plates " of reptiles ?) also; as there are no difficulties about the latter which are not also presented by the teeth.
There appear, in fact, to be but few ob jections of any importance to the assump tion of the ecderonic nature of fish scales, the principal ones being the continuation of the tissue of the ecderon over the upper surface of the scales; the apparent passage of the bony structure into the laminm of the connective tissue of the enderon below, and the vascularity of the latter.