366 TEGUMENTARY ORGANS 



aware, no elucidation of this point has hitherto been undertaken, and 

 yet, for want of it, the greatest confusion prevails in the nomenclature 

 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 

 calcareous salts. 



Following Mr. Bowman, — who, of course, when he wrote his 

 well-known article on " Mucous Membrane," in this Cyclopaedia, could 

 not contemplate the new questions to which the progress of ten years 

 would give rise, — many 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 where we most want it ; for 

 among the lower animals, and in some integumentary organs among 

 the higher, membranes identical in structure, or rather in structureless- 

 ness, with " basement " membranes, 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 the germinal disc is once formed? It is a separation 

 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 arese 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 serotis layer, the primordial 

 tegumentary system ; while the inner is the mucous layer, the 

 primordial viscus. Of course each of these, while actually integument 

 and intestine, re-presents potentially a great deal more, — the former, for 

 instance, in the higher animals becoming eventually differentiated into 



