476 



TEGUMENTARY ORGANS, 



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 areae of growth and metamorphosis, 

 I term the Protomorphic 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 Tseniadae, 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 

 protomorphic 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 protomorphic (or 

 cambial} line, from which metamorphosis into 

 periderma goes on outwards, while inwards it 

 passes into, 3rd, the metamorphosed tissue of 

 the mesophloeum. Next to this comes, 4th, 

 the metnmorphic area of the enclophlceum 

 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. 



I have endeavoured to render these relations 

 obvious by the diagram (fig. 304.), which may 

 be taken for a section from centre to surface 



of a foetal limb, or of an exogenous branch, a, 

 outer protomorphic line between epidermis or 

 periderma and mesophloeum in the plant ; 



Fig. 304. 



between ecderon and enderon in the animal ; 

 a', inner protomorphic line between liber and 

 wood of plant, between bone and periosteum 

 of animal ; 6, 6', cork and epidermic layers of 

 plant ; cellular epidermis and scale of animal, 

 fish, e.g. ; c, mesophloeum, enderon (derma) ; 

 </, liber, periosteum ; e, e' t wood and pith, bone 

 and cartilage ; x, axis; */, surface. 



The consideration of vegetable structures 

 will aid us even further in understanding the 

 manner in which the different varieties of in- 

 tegumentary organs, with which we shall meet, 

 are formed. For it is well known that the 

 outer covering of a plant may ultimately be 

 constituted in one of three ways. 1. The 

 original cellular ecderon may persist un- 

 changed. 2. The " epiderm " persisting, a la- 

 minated, but otherwise structureless " cuticula" 

 may be developed upon its outer surface, 

 attaining sometimes a very considerable thick- 

 ness. 3. The original epidermis is cast off, 

 its place being taken by the development of a 

 new layer of different, usuallj' suberous con- 

 stitution, beneath it, which then goes on 

 growing endogenously, and constitutes the 

 permanent integumentary surface. Now, we 

 find a precise parallel for all these conditions 

 in animals. In the soft integument of most 

 Mollusca and Vertebrata the first condition 

 obtains, the general surface of the integument 

 being constituted by the cellular "epidermis." 



In the Annulosa, on the other hand, the 

 integument has certainly, in many cases, and I 

 think probably in the great majority, the cha- 

 racter of a vegetable cuticle, consisting as it 

 does of layers developed from the outer sur- 

 face of the cellular ecderon. In this way also 

 I believe that all molluscan shells are formed. 



Lastly, the fish-scale produced altogether 

 beneath the cellular ecderon or epidermis, 

 but growing endogenously after the manner 

 of a true ecderonic structure, appears to be 

 precisely analogous to the corky periderma 

 of the plant ; and as the latter, though it is 

 not the original epidermis, takes its place and 

 grows in the same way, so in the fish the 

 scale, which is assuredly not a calcification of 

 the cellular ecderon, yet represents it both in 

 position and in mode of growth. 



2. Morphology of the integuments. In the 



