TOOTH-GENESIS IN THE CAVIIDA. 271 
tooth, separating the internal cone from its base; and as this 
detached portion is fused by its apex with the external cone, the 
tooth in section has the appearance of an inverted V, the 
external limb of which is considerably longer than the internal. 
How this communication is brought about, whether by the 
rupturing inwards of the central cavity, or by the extension into 
the latter of a channel running outwards from the internal 
surface, I am not in a position to say. 
If the condition of this tooth be traced still further backwards, 
the communication is still seen to be present ; and, in addition, 
the central cavity communicates with the surface, the apices 
of the external and internal cones being separated. This may 
have been brought about by the formation of a cleft from with- 
out inwards, or from within outwards ; or, what I think the more 
probable is, that the apices of the two cones have here remained 
separate, not having undergone fusion, as in every instance 
the posterior moiety of the tooth seems to be in a somewhat 
earlier stage of development than the anterior. 
Ina section through this region, the apex of the internal cone 
Fig. 4. 
sues 
Diagrammatic sections through Third Upper Cheek-tooth. A from Stage 2. 
B, C, D, H, from Stage 3. 
lies as an isolated mass to the inner side of the external, the 
latter retaming its connection with the fused bases of the two 
cones, appearing almost identical in section with that through 
the centre of the deciduous tooth. This I regard as being a 
