TEGUMENTARY ORGANS 369 



metamorphosis proceed in the same way. The result is, of course, the 

 division of the germ into three laj-ers — a central and two superficial 

 (inner and outer) planes of metamorphosed tissue — and two planes 

 whence growth and metamorphosis proceed. 



It results from all this, that, among the higher animals, the true 

 homologue of the integument of the Hydra is the epidermic layer 

 alone. But it would be exceedingly inconvenient to change the 

 accepted meaning of " Integument " on this ground ; and, therefore, 

 I shall, throughout the present article, consider as integument — the 

 outermost plane of indifferent tissue in tlie animal body, ivitli its external 

 and internal area of metamorpJiosis collectively ; these being simply the 

 expressions of two processes of groivth in opposite directions, and their 

 line of contact. 



It must not be supposed that this phraseology 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 expression 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 structure of an organ ; so that to speak of" Epidermic " 

 bony or fibrous tissue would sound almost contradictory. Again, all 

 these distinctions, which have been shown to exist between the two 

 elements of the integument, equally hold good with regard to the 

 mucous membranes. Now we have a term " Epithelium " for the 

 epidermic element of the latter ; but there is, as far as I know, none 

 for the element which corresponds with the derma. Nor have we any 

 word for the boundary line between the endogenous and exogenous 

 ares 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 metamorphosis. 



The boundary line — passing through indifferent tissue — between 

 any two such opposite arese of growth and metamorphosis, I term the 

 Protomorphic line. The whole external (free) area of metamorphosis 

 I call the Ecderon ; the entire internal (deep) area of metamorphosis, 

 the Enderon. 



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