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 bulk 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 Nasmyth’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 with 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 
