TEGUMENT ARY ORGANS. 



481 



teeth, and thence with the teeth of all verte- 

 brata. Again, it appears to me indubitable 

 that the teeth and the hairs are homologous 

 organs; they are therefore either both en- 

 deronic or both ecderonic. Taking for granted 

 the validity of a basement membrane as a 

 mark of the boundary between ecderon and 

 enderon, I elsewhere* arrived at the conclu- 

 sion that the teeth are enderonic organs, and 

 that therefore the hairs must follow them. 

 Now, however, that a " basement membrane " 

 turns out to be no test at all, there seems no 

 reason why we should not be guided entirely 

 bv the direction of growth, and consider both 

 hairs and teeth as ecderonic organs ; the 

 former being a development of the cellular 

 ecderon, and corresponding with the ordinary 

 horny epidermis ; the latter, a development 

 of a deep layer of the ecderon beneath this. 

 It appears to me that we can do no other 

 than admit this view for the teeth ; but if this 

 be the case, we may apply it to the scales of 

 fish (and the " dermal plates " of reptiles ?) 

 also; as there are no difficulties about the latter 

 which are not also presented by the teeth. 



There appear, in fact, to be but few ob- 

 jections of any importance to the assump- 

 tion of the ecderonic nature of fish scales, 

 the principal ones being the continuation 

 of the tissue of the ecderon over the upper 

 surface of the scales; the apparent passage of 

 the bony structure into the laminae of the 

 connective tissue of the enderon below, and 

 the vascularity of the latter. 



The continuity of the enderon over the 

 scales will be seen below to be more apparent 

 than real. I have not been able entirely to 

 satisfy myself, as to the exact relations of 

 the parts, in the case of the eel, but in 

 the other fishes which I have examined the 

 surface of the scale is very partially covered 

 by the enderon, being in its centre, at any rate, 

 in'contact with the cellular ecderon. 



The vascularity of the scale never extends 

 to its most superficial layers, and may be ex- 

 plained in the same way as that of the test of 

 an Ascidian, which however is unquestionably 

 an ecderonic structure. The passage of its deep 

 layers directly into the connective bundles of the 

 enderon, which Ley dig has observed in Poly- 

 pterus (and which I will not sayjdoes not occur 

 elsewhere, though I have not observed it), 

 would appear to me only to indicate that this 

 scale, and perhaps others, are composed of two 

 portions, a superficial ecderonic part extend- 

 ing as far as the most superficial vascular 

 canals, and a deep portion beneath these be- 

 longing to the enderon. 



However, all these points can only be de- 

 cided by a much more extensive series of in- 

 vestigations, principally directed to the ascer- 

 tainment of the position of the protomorphic 

 line and of the direction of growth of the 

 constituents of every scale, than I have hither- 

 to had time or opportunity to carry out ; 

 and as the attention of other observers does 



* On the Structure and Development of the 

 Teeth, Quarterly Journal of Micros. Science, 1852. 

 Sup p. 



not appear to have been directed to these par- 

 ticular points, the question must for the 

 present remain undecided. 



Professor Williamson in his valuable and 

 philosophical contributions to our knowledge 

 of this subject (Phil. Trans. 1849-1852) 

 laid the foundation for a comprehension of 

 the mode of development of fish-scales, by 

 pointing out that Agassiz's views, though 

 essentially true, yet require a certain modifi- 

 cation. For though a fish-scale does really 

 grow by the apposition of layers to its deep 

 surface, as Agassiz asserted, yet it is not in- 

 cluded in a sac of the epidermis (if by that 

 term we are to understand the ordinary 

 cellular ecderon) ; and it is also true that its 

 deeper portions grow by their superficial 

 surface. Professor Williamson points out, in 

 fact, that every fish-scale consists of at least 

 two portions, a superficial homogeneous, or 

 at most canaliculated, laminated layer, the 

 ganoin (so called enamel or horny layer of 

 authors), and a deeper, also laminated, fre- 

 quently fibrous or osseous portion commonly 

 traversed by Haversian canals. Now these 

 two portions have a certain independence in 

 their mode of growth, at any rate after their 

 first formation, as may be easily understood by 

 the accompanying diagram {fig. 307.), which 

 represents a series of imaginary sections of 

 scales from their first growth onwards ; a, is 

 the protomorphic plane; 6, 6", the deep ecde- 

 ron ; 6', the superficial cellular ecderon, and the 

 line x, the centre of the scales from which 

 development commenced. 



Kg. 307. 



Suppose A to be the youngest scale, con- 

 stituted merely by a thickening and calcifica- 

 tion of the deep ecderon, which in u has 

 added several layers by apposition to its inner 

 surface, all of which retain the ganoin struc- 

 ture except the deepest, which becomes fibrous 

 in its texture, and forms the commencement 

 of the " Lepidine" layers of the scale ; these 

 layers, however, being as much a part of the 

 ecderon as the former. In c the scale widen- 

 ing, the edges of its " Lepidine " layer do not 

 remain in contact with the ganoin layer; but it 

 will be obvious that the re-entering angle 

 thus formed by the protomorphic line between 

 the two, is only, as it wtre, a fold of the deep 

 surface. If the two layers go on increasing 



