380 TEGUMENTARY ORGANS 



fishes are homologous organs ; nor as less so that the tegumentar)^ 

 spines of the Plagiostomes are homologous with their teeth, and 

 thence with the teeth of all vertebrata. Again, it appears to me 

 indubitable that the teeth and the hairs are homologous organs ; they 

 are therefore either both enderonic 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 conclusion 

 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 by 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 objections of any importance 

 to the assumption 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 explained 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 Leydig has observed in Polypterus (and which 

 I will not say does 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 



' On the Structure and Development of the Teeth, Quarterly Journal of Micros. Science, 

 1852. 



