16 GENERAL ANATOMICAL CHARACTERS 



constituent of the perfectly-formed tooth. Below the mass of epi- 

 thelial cells thus embedded in the substance of the gum, and remaining 

 connected by a narrow neck of similar structure with the epithelium 

 of the surface, a portion of the vascular areolar tissue becomes 

 gradually separated and defined from that which surrounds it, and 

 assumes a distinct form, which is that of the crown of the future 

 tooth, — a single cone in the case of simple teeth, or with two or 

 more eminences in the complex forms. This is called the dental 

 papilla or dentine germ, and by the gradual conversion of its tissue 

 into dentine the bidk of the future tooth is formed, the uncalcified 

 central portion remaining as the pulp. The conversion of the 

 papilla into hard tissue commences at the outer surface of the apex, 

 and gradually proceeds downwards and inwards, so that the form of 

 the papilla exactly determines the form of the future dentine, and 

 no alteration either in shape or size of this portion of the tooth, 

 when once calcified, can take place by addition to its outer surface. 

 In the meanwhile, calcification of a portion of the cells of the enamel- 

 organ, which adapts itself like a cap round the top of the dentinal 

 papilla, and has assumed a somewhat complex structure, results in 

 the formation of the enamel -coating of the crown of the tooth. 

 While these changes are taking place the tissues immediately sur- 

 rounding the tooth-germ become condensed and differentiated into 

 a capsule, which appears to grow up from the base of the dental 

 papilla, and encloses both this and the enamel-germ, constituting 

 the follicle or tooth-sac. By the ossification of the inner layer of 

 this follicle the cement is formed. This substance, therefore, unlike 

 the dentine, increases from within outwards, and its growth may 

 accordingly be the cause of considerable modification of form and 

 enlargement, especially of the roots, of certain teeth, as those of 

 Seals and some Cetacea. The delicate homogeneous layer coating the 

 enamel surface of newly-formed teeth, in which cement is not found 

 in the adult state, and known as Xasmyth's membrane, is considered 

 by Tomes as probably a film of this substance, too thin to exhibit 

 its characteristic structure, though by others it is believed to be 

 derived from the external layer of the enamel-organ. The homology 

 of the teeth Avith the dermal appendages, hairs, scales, and claws, 

 has already been alluded to, and it will now be seen that in both cases 

 two of the primary embryonic layers are concerned in their develop- 

 ment — the mesoblast and epiblast — although in very different pro- 

 portions respectively. Thus in the hair or nail the part derived from 

 the epiblast forms the principal bulk of the organ, the mesoblast 

 only constituting the papilla or matrix. But in the tooth the epi- 

 blastic portion is limited to the enamel, and is always of relatively 

 small bulk and often absent, while the dentine (the principal con- 

 stituent of the tooth) and the cement are formed from the mesoblast. 

 "When more than one set of teeth occur in mammals, those of 



