TEGUMENTARY ORGANS. In en deavouring to deal with so large a subject as the tegumentary organs of animals, within the limits of an article like the present, it ap peared advisable not to attempt to enter into minutim of detail (which indeed fall more properly within the province of those who treat of the special classes), but so far as pos sible to regard these organs as a system in the senSe of Bichat —as a sort of zoological class —whose meinbers, the tegumentary organs of particular animals, are but special modifica tions of one general plan. In reflecting how this might best be done, however, I was met at the outset by certain difficulties and per plexities whose solution appears to me to be essential to any philosophical treatment of the subject, and to the consideration of which I, therefore, propose to devote the following Preliminaly Section.
SS 1. My first difficulty was to find an answer to the question,—What constitutes a tegu mentary organ as distinguished from any other ? The most obvious definition of an integu ment or tegunzentary organ is, of course, — that which forms the external covering of any animal— viscus, on the other hand, being that which is contained. More strictly, it may be said that the integument constitutes that free surface of an animal which is ex ternal to the edges of the oral and anal aper tures, or where the former alone exists, to its edge. Now these definitions are perfectly sufficient so far as surface is concerned ; but suppose we make a section perpendicular to the surface, where does integument cease, and where does viscus begin ? So far as I ain aware, no elucidation of this point has hither to been undertaken, and yet, for vvant of it, the greatest confusion prevails in the nomen clature of those organs which constitute the outer wall of the animal frame.
Intimately connected with this question, and indeed forming a part of it, is a second. In man and the higher animals, there is an universally recognised distinction of the integument into two portions,— the epidermis and the derma; and these terms have been extended to all animals. But, if we inquire what constitutes an epidermis, and what a derma, no definite answer is to be met with. It may be said that the derma is vascular, while the epidermis is nonvascular ; or that the epidermis is a simple cellular horny structure, while the derma is complex and fibrous ; but these characters, applicable enough among the higher animals, fail completely with the lower.
Thus, in the majority of the Invertebrata, the derma cannot be said to be vascular, while, on the other hand, the epidermis, or its representative, assumes the structure of fibrous tissue, bone, cartilage, dentine, and enamel, — acquires, in fact, the utmost complexity, and, instead of possessing a horny nature, contains chitin, cellulose or calca reous salts.
Following Mr. Bowman,— who, of course, when he wrote his well-known article on " Mucous Membrane," in this Cyclopmdia, could not contemplate the new questions to which the progress of ten years would give rise,—rnany regard that which is external to a " basement membrane " as epidermic, that which is internal to it, as dermic structure. This test, however, fails us %%here we most want it ; for among the lower animals, and in some integumentary organs among the higher, tnembranes identical in structure, or rather in structurelessness, with " basement " tnem branes, may be met with, forming the surface of' what are assuredly epidermic organs.
I believe that here, as elsewhere, the only ultimate appeal lies to development, both as it occurs in the embryo and as it goes on in the adult. What, in fact, is the first process which takes place in the embryo, when tl,e germinal disc is once formed ? It is a sepa ration into two layers, by the setting up within the outer portion of the primitive germ of a process of growth independent of that in the inner portion. Where these two arem or planes of growth, as they might be called, meet, the germ readily separates into two portions, the outer of which is the so-called serous la.yer, the primordial tegumentary system ; while the inner is the mucous layer, the primordial viscus. Of course each of these, while actually integument and intestine, represents potentially a great deal more,— the former, for instance, in the higher animals becoming eventually differentiated into the proper tegumentary system and a great part of the nervous, the muscular, and the vas cular systems ; but what I wish to direct attention to at this moment, is the fact, that this first differentiation into integument and viscus proceeds from the setting up of two independent lines, or rather planes of growth, in the germinal membranes.