TEGUMENTARV ORGANS 379 



partitions from the apex of the pulp, as the quill js pushed beyond 

 it ; thus protecting itself from the air admitted by the umbilical 

 aperture, and which is visible, occupying the chambers thus formed 

 {fig. 316. G). 



There can be no question as to the relations of the integumentary 

 organs hitherto described to the primary constituents of the integu- 

 ments, but it is different with regard to those calcified tegumentary 

 appendages, the scales of Fishes, and the so-called " dermal" calcified 

 plates of Reptilia and Mammalia. One point is quite certain with 

 regard to these appendages, that they are not, like the calcified shells 

 ■of the moUusca, the representatives of the outer portion of the 

 originally cellular epidermis (are not therefore comparable to the 

 ■" cuticula " of a plant), inasmuch as the latter may always, in their 

 young state, be traced over them. It is for this reason, I imagine, 

 that they are at present ordinarily called " dermal " organs. A truly 

 dermal or enderonic organ, however, ought, if it continues to grow, 

 to retain the same characters as the enderon of which it forms a part. 

 It ought, therefore, to have its protomorphic surface external and to 

 grow exogenously. Now, no scale or plate of any fish, so far as I am 

 aware, does this ; on the other hand, it holds good of all, whether 

 Placoid, Ganoid, Cycloid or Ctenoid,^ that they commence by the 

 ■occurrence of a calcific deposit immediately beneath the cellular 

 ■ecderon, and that they increase by continual addition to the inner 

 surface of this primary deposit. There are two ways in which we 

 may conceive that these scales and plates are produced. Either they 

 are a gradual calcification of the whole enderon from without inwards 

 ■(which is the view taken by Leydig, of the scales of Polypterus), in 

 which case the only tissue of the enderon capable of increase (that 

 •of the protomorphic line) being arrested by the calcareous deposit, 

 the whole enderon at these parts must cease to grow, which would 

 appear to be contrary to fact ; or the scale corresponds with the 

 ■cork-layer of the vegetable integument, and like it, though developed 

 beneath the ordinary cellular epidermis, is still a truly ecderonic 

 structure. 



A great deal might be said for both these views ; and if in this 

 place, I assume the latter to be more correct, it is because I think we 

 must be guided by the homology of the scales with certain other 

 organs, where these relations are more definitely expressed. It may 

 be taken as certain, I think, that the scales, plates, and spines of all 



' And I believe it will be found to be equally true of the " dermal " bones of reptiles and 

 mammals. 



