OF THE NEWT, EROG, SLOWWORM, AND GREEN LIZARD. 
291 
teeth only just the extreme tips project beyond the surface of the epithelium, so that 
their functional importance can be but small. 
On its inner side the area is, as in the newt, bounded by a connective-tissue frame- 
work only (figs. 12, 13, & 14), which is in appearance very different from the tissue 
occupying the residual space within the area. The tooth-sacs themselves differ but 
little from those of the newt, though the columnar character of the cells composing 
the inner layer of the enamel-organ is less strongly marked*. 
The connective tissue which is in the neighbourhood of a forming tooth-sac becomes 
to some extent arranged round it, though nothing amounting to a definite connective- 
tissue capsule is formed ; indeed I have never been able to satisfy myself of the existence 
of a membranous investment to these sacs, though I would not go so far as to deny its 
existence. 
The first step towards the formation of a sac is that inflection of the oral epithelium 
(f in figs. 12, 13, 14) which ultimately forms the enamel-organ; but the connexion 
between this latter at the apex of the sac with the oral epithelium is not long trace- 
able, for the tooth-sac is so close beneath the surface, that it comes to be in contact 
with the epithelium along a considerable part of its circumference (figs. 11 & 13). 
In close relation with the inner boundary of the tooth-sac is to be found the enamel- 
germ for the successional tooth-sac (f, fig. 13); but whether this arises directly from 
the epithelium, starting anew, as it were, for the formation of each tooth, or is derived 
from the cells going to form its predecessor, is very difficult to determine, as the 
migration of the growing sac speedily masks its origin and would destroy any such 
connexion. Hence the enamel-germs often stand alone, as in figs. 14 & 12 ; and 
appearances lead to the supposition that their origin is quite independent of previous 
tooth-germs. 
After the tooth has attained to nearly its full size and is displacing its predecessor, 
the formative dentine-pulp undergoes change ; the distinct character of the odontoblast 
layer is lost, and it becomes metamorphosed into a close-meshed connective-tissue 
reticulum, poor in vessels, a single vascular loop being usually all that it presents 
(fig. 11). The tooth becomes attached to the bone more securely than that of the 
newt, for it is mounted on a more complete pedestal (fig. 11), and not merely soldered 
on by its outer edge ; the inner buttress of bone ( d in fig. 11) is not, however, complete, 
but is perforated to admit vessels, and also often excavated by the successional tooth-sac. 
In the frog, therefore, just as in the newt, there is no such thing as a dental groove, 
no such thing as free dental papilla, and no process of encapsulation such as Goodsir 
conceivedf. 
* If any enamel at all is formed, it is only an exceedingly thin layer. Prof. Owen described an investment 
of enamel on the convex surface only, and a layer of cement on the concave surface, a distinction in which I 
am unable to follow him. Waldeyeb says that Owen is altogether mistaken in supposing that the frog has 
any enamel at all, while Prof. Hxtxeey speaks of the existence of an exceedingly thin layer of enamel, 
t Professor Owen (Odontography, p. 185) writes : — “ In the frog the dental germ makes its appearance in 
