10 



NATURAL HISTORY OF VERTEBRATES. 



Figs. 7 and 8. — Embryonic and mature teeth in skin 

 of a mailed siluroid ; c, cement bone developed 

 as a socket for the tooth ; d^ dentine ; e, enamel ; 

 eo, enamel organ ; od, odontoblast ; p^ papilla. 



ular the modifications which they assume. The most primitive exoskeletal structures 

 are the little teeth which heset the skin in sharks, constituting the ' shagreen,' derived 

 from them ; similar teeth occur in the skin of the sturgeon between the larger bony- 

 plates, so characteristic of that fish. The mode of formation of such teeth may be 

 readily understood from Figs. 7 and 8, which illustrates the development of such 



dermal teeth in one of the mailed siluroids. 

 The seat of formation is obviously one of the 

 papillsB of the cerium, but certain connective 

 tissue cells clothing the papilla (^od) have 

 the property of secreting a substance which 

 becomes impregnated with salts of lime, and 

 simultaneously of leaving some of their own 

 processes in the hard substance so formed. 

 This is dentine, and the forming cells are 

 termed odontoblasts. The surface of the 

 young tooth becomes coated with a cap 

 of very hard enamel (e), which is secreted 

 from the lower columnar layer of epidermal 

 cells, modified at this place into a so-called enamel organ. Eventually the tooth grows 

 out beyond the level of the epidermis, the papilla remains as the pulp cavity of the 

 tooth, and all trace of the epidermal cells are soon rubbed off from the enamel cap. 

 Deeper down in the corium a sort of socket is formed for the tooth by the formation 

 from the connective tissue of a small plate of bone, so-called cement bone. These 

 structures are of great interest and importance, because the bony plates of the stur- 

 geon, already referred to, are simply formed by the fusion of such cement plates, and 

 even the rhomboid scales of the gai-pike have a similar origin, although they have lost 

 the asperities which characterize the plate of the sturgeon. The various forms of 

 scales met with in the Teleosts are developed in the corium in a similar way, but in 

 the ordinary ' cycloid ' and ' ctenoid ' scales, there is never any enamel or dentine 



formed on their surface. We have noted above that sim- 

 ilar bony plates may be also formed for the protection of 

 the sense organs lodged in the corium, and it is necessary 

 to call attention specially to these two forms of dermal 

 bones, as we shall find them later entering into such 

 important relationships with the endoskeleton, that we 

 shall have to treat of them in that connection. Few exo- 

 skeletal structures occur in the Amphibia. Crocodiles, on 

 the other hand, are well provided with such plates, but 

 Fig. 9.— Section through the oarapax they nowhere reach such a high degree of development 

 mis"; mj^m^u'oous layer!"' "' ^^^ ^^' as in the turtles, where they form a more or less complete 



box for the protection of the animal. Many of the plates 

 developed in the corium in this way enter into close union with parts of the 

 endoskeleton, others are quite free from such connection, while all are closely en- 

 veloped by the horny scales of epidermal origin ; in many a thin strip of unossified 

 corium and the mucous layer of the epidermis intervening between them (Fig. 9). 

 Few mammals are possessed of dermal bones of similar origin to those of the 

 turtles, but the living and extinct ai-madillos of South America form an exception to 

 this rule. 



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