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ME. C. S. TOMES ON THE DEVELOPMENT OF THE TEETH 
term the “ area of tooth-formation,” inasmuch as it contains nothing but structures con- 
cerned in the development of teeth. 
Its outer limit has already been mentioned ; on the inner side, toward the median line 
of the palate, it has no osseous boundary, but it is nevertheless very sharply defined by 
connective tissue (e, figs. 1, 2, 3, & 5). 
At the surface, where it is continuous with the epithelium of the mouth, it is narrow ; 
but as it becomes deeper it widens, so that the whole area* is roughly triangular in 
form, as is seen in figs. 2 & 3. 
Along its basal or deepest portion, nearly, though not quite, resting upon the bone, 
are ranged, in horizontal series, two, three, or even four tooth-sacs, the youngest lying 
nearest to the middle line. A connexion between the apices of the sacs and the epi- 
thelium of the surface may be traced with more or less distinctness in every section 
through an elongated narrow neck of cells f; to the inner side of the youngest tooth- 
sac may also generally be seen a csecal process of epithelial cells (f \ in figs. 2 & 5), and 
to the inner side of this again another and shorter epithelial process, which does not 
extend so deeply (f in figs. 2 & 5). 
The individual tooth-sacs are oval, very slightly flattened at their bases, and sharply 
defined ; when it is so viewed that its surface is in focus, this is seen to be made up of 
a tesselated epithelium of great regularity, and when it is subjected to pressure it breaks 
up into a mass of cells $ and nothing else (fig. 7). 
The arrangement of the cells in the tooth-sac appears to have escaped the notice of 
previous writers, though it is to some extent analogous with that met with in Mammalia : 
there is a dentine-papilla, the cells upon the surface of which are arranged in ah <! odon- 
toblast ” layer (figs. 4 & 8) ; and outside this papilla, which is very soon capped with 
dentine, comes a layer of columnar epithelial cells, similar to the enamel cells or internal 
epithelium of the enamel-organ of mammals. At the base of the dentine-papilla this 
layer of columnar cells becomes continuous with a second layer of shorter cells, which 
lie externally, and constitute the tesselated epithelium already mentioned as forming 
the surface of the sac (figs. 4, 5, 6). The “ enamel-organ” is therefore, like that of the 
armadillo, made up wholly of the two layers of cells, without any intermediate tissue. 
The continuity of the cells constituting the enamel-organ with the epithelial processes 
or necks before alluded to can generally be traced (figs. 1, 2, 3, 5, 6). 
The base of the dentine-papilla is sharply defined, and no crescentic processes pass 
up from it around the outside of the enamel-organ, to take a share in the formation of 
* The upper jaw has been selected for description because the tooth-sacs are less crowded together than 
in the lower jaw. 
t This was mentioned by Dr. Lionel Beale, who, however, did not trace out its developmental origin ; and 
it was observed also by Professor Huxley in the tooth-sac of the mackerel. 
j Although there may be some theoretical difficulties in the way of accepting this, I am, after repeated 
examination, inclined to concur in the opinion very positively expressed by Dr. Beale, that these sacs have no 
limiting membrane whatever. 
