370 TEGUMENTARY ORGANS 



It will be observed that these definitions rest wholly upon the 

 mode ofgroivth, and leave altogether out of consideration the structure 

 of the resulting tissue. In fact, as I have already said, an extensive 

 study of the integumentary 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 homogeneous 

 membrane, as in the Taeniadse, 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 considerations that they enable us to carry still 

 further the doctrine of the identity of structure of plants and animals 

 sketched by Caspar Wolff, and developed in our own times by 

 Schwann. If we make a transverse section of the growing limb of a 

 vertebrate animal, leaving out of consideration, for the moment, the 

 vessels, nerves, and muscles, we observe from the surface inwards, ist, 

 the ecderonic area of metamorphosis ; 2nd, the integumentary 

 protomorphic line ; 3rd, the enderonic area of metamorphosis ; 4th,, 

 the periosteal area of metamorphosis ; 5 th, 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 metamorphosis 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 peridermal 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 metamorphic area of the endophloeum or liber ; within which is,. 

 5th, the protomorphic line of the cambium, which becomes meta- 

 morphosed on its inner surface into, 6th, the wood ; within which lies, 

 7th, the pith, the result of the metamorphosis of the primitive 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 ;, 

 between ecderon and enderon in the animal ; «', inner protomorphic 



