290 
ME. C. S. TOMES ON THE DEVELOPMENT OP THE TEETH 
sharply defined and mapped off from surrounding tissues from the very first. The 
dentine-papilla gives rise to no prolongations from its base ; but the whole tooth-sac is 
at first nearly spherical (figs, 6, 7, & 8), and I have failed in any section to clearly see 
that the dentine-papilla has an origin distinct from the enamel-germ. Nevertheless 
the close resemblance borne by the completed tooth-sac of the newt, as well as the 
identical relations displayed by its enamel-germ, to that of the Anguis fragilis and 
green lizard, in which I have succeeded in tracing the whole process, as well as the 
prima facie improbability of such a view, leads me to reject the views advanced by Dr. 
Lionel Beale and Leydig, that the whole tooth, including the dentine, is derived from 
an epithelial origin'*. 
Common Frog . — The general features of the process are closely similar to those 
observed in the newt, although there are many differences of detail. 
The region designated as the area of tooth-development, which extended far into the 
palate in the newt, is very circumscribed in the frog, so that there is not room for 
more than one successional tooth-sac at one time (Plate 47. figs. 11, 12, 13). 
And instead of the successional tooth-sac attaining to a considerable size without 
noteworthy encroachment on neighbouring structures, it obtains space and at the same 
time protection by the absorption of a portion of the bony parapet carrying the teeth 
or of the tooth itself (see figs. 12 & 13); thus it is not very unusual for the whole 
tooth-sac to pass bodily into what corresponds to the pulp-cavity of the tooth already in 
place (fig. 12). 
This recession of the tooth under some shelter is in a measure a necessary consequence 
of the peculiar antagonism of the upper and lower jaws. 
The lower jaw has a smooth rounded border and no vestige of a lip; when the 
mouth is closed it passes not only within the upper lip, but also within the teeth and 
their supporting parapet of bone (see diagrammatic section, fig. 10), and is received into 
a groove, which it closely fits, formed between the maxillary parapet and an inward 
jutting process which fits beneath the very peculiar tongue (figs. 10 & 11). Of the 
* Dr. Lionel Beale says, “ The tooth is not developed from a papilla, consisting of snbbasement tissue, hut 
it is formed in the very centre of a collection of cells ; and it is clear that these cells have been formed in the 
central part of a preexisting cellular mass, so that the oldest colls, •which seem but to perform the office of a 
protecting envelope, are outside, and, as new ones have been produced in the centre, these oldest cells have 
become somewhat flattened on the surface, thus giving the appearance of a boundary or imperfect capsule, 
which enables us to distinguish these masses from the collection of cells in which they are imbedded. 
“ I have seen a single cell, differing from its neighbours in its larger size, dividing to form three or four 
separate cells ; and I believe this was the original cell from which all those which constitute the collection in 
which the tooth at length appears resulted.” 
In this account neither the intimate structure of the sac nor the share taken by the dipping inwards of 
the epithelium is mentioned ; nor was Leydig more explicit in his descriptions. 
What is meant by Santi Sirena in the statement that the tooth of the newt is developed free in the mucous 
membrane, I do not exactly know; but it is clear that he cannot have recognized the very definite structures 
which exist, or he would hardly have so expressed himself. 
