OF THE NEWT, FEOGr, SLOWWOEM, AND G-EEEN LIZAED. 
293 
blast layer ( o in fig. 22) is very clearly to be seen ; and where it has been accidentally dis- 
placed to a slight extent, the dentinal fibrils discovered in the human tooth by my father 
may be seen like harp-strings stretching across to the dentine (fig. 22). Beneath the 
odontoblast layer comes an areolar tissue framework, much like that which occurs in 
mammalian tooth-pulps. Thus in the teeth of the lizards the tooth-pulp attains to a 
higher organization, and is less soon converted into a mere connective-tissue reticulum, 
than in the newt and frog ; and although we have no actual basis of observation to rest 
upon, it is therefore highly probable that the durability of each individual tooth after 
it has become attached to the jaw is greater. As the tooth moves up into position the 
whole of the structures comprised in the tooth-sac, including the outer loose and ill- 
defined investment of connective tissue, go with it. 
When its outer border reaches the level of the top of the alveolar parapet (as in figs. 
16 & 21) it comes into contact with a tolerably well-defined band of connective tissue, 
which runs up from the apex of the bone towards the epithelium of the surface (m in 
fig. 21), and, when there is no tooth in place, bounds the area of tooth-formation on its 
outer side. This is continuous with the periosteum, and probably plays an active part in 
securing the tooth to the bone ; it may be invariably recognized when a tooth is nearly in 
place, and was seen by Professor Huxley, who mentions that a membrane may be traced 
on to the tooth of the frog from the outer surface of the bone. The precise manner in 
which the succession and attachment of the teeth is effected is a matter of much interest, 
but is rather beyond the scope of the present communication. 
The enamel-organ with its double layer of cells remains distinctly recognizable up to 
the time when the tooth comes into position on the bone ; as it does not quite reach 
to the base of the dentine-papilla (see fig. 21), it does not intervene between the dentine 
and the apex of the bone and its periosteum ; it is lost sight of afterwards*. 
On the inner side the characteristic folding over of its cells, where the inner merges 
into the outer layer, may be seen after the tooth is in place, closely applied to the surface 
of the tooth (see fig. 18). 
As this row of cells intervened between the dentine and the capsule, it is quite 
certain that the tooth cannot have received any investment from the ossification of the 
capsule. 
Before any generalizations can advantageously be drawn from these or any other 
observations, the subject of the development of the teeth in Fishesf requires further eluci- 
dation ; and some investigations which I have commenced in that direction are not as 
yet sufficiently extensive to serve as a basis for general statements. The tooth-sacs of the 
Anguis fragilis and Lacerta viridis are, however, instructive, inasmuch as they are deve- 
* My preparations do not enable me to speak with absolute certainty as to the ultimate disposal of the enamel- 
organ ; the point requires further investigation. 
t The only reliable description of the tooth -sac of a fish with which I am acquainted, is given by Professor 
Huxley in the paper already several times quoted. 
MDCCCLXXV. 2 E 
