ON THE DEVELOPMENT OF THE TEETH 239 



in any way to the development of the dental tissues, though they may 

 indirectly. 



3. The histological elements of the pulp take no direct part 

 (except, perhaps, eventually in the cement) in the development of the 

 dental tissues, becoming either absorbed or being pressed in by the 

 gradual increase- of the latter. The Conversion Theory is, therefore, 

 as incorrect as the Excretion Theory, and the dentine is formed, not 

 by ossification of the histological elements of the pulp, but by 

 deposition in it, " parenchymate materiam suppeditante." 



I have already exceeded my limits, and I must, therefore, dismiss 

 my last point very concisely. The true homologues of the teeth in Man 

 are, I think, the Hairs. As Hildebrandt says, " As the Hairs in their 

 bulb (sac), so the Teeth are developed in their capsules." The stage 

 of the free papilla, which does not occur in the hairs of man, is absent 

 in the teeth of the Mackerel and Frog, and, indeed, it would seem in 

 the permanent dental capsules of man also. 



Substitute corneous matter for calcareous, and the Tooth would be 

 a Hair. The cortical substance of the hair contains canals not unlike 

 those of the dentine ; its relation to a dermal papilla is the same as 

 that of the dentine : ^ for although it is universally stated to be such, I 

 think it can be shown that the hair shaft is not an epidermic structure, 

 but a dermic one. 



Again, the so-called cuticle of the hair corresponds in all respects, 

 except absolute and relative size, with the enamel — its inner layer 

 with the enamel proper — its outer with Nasmyth's membrane. On 

 the root of the hair the cuticle is not continuous with the proper 

 ■epidermic cells, but with a structureless membrane, which occupies 

 more or less distinctly the place of a meuibrana preforinativa. The 

 two root-sheaths, again— true epidermic structures, but which do not 

 enter all into the construction of the hair proper— represent the 

 altered and unaltered portions of the " enamel-organ!' 



Hairs and Teeth, then, are organs in all respects homologous, and 

 true dermal organs. Under the same category, probably, will come 

 Feathers and the Scales of fishes. 



The Nails, on the other hand, seem to be purely epidermic, at least 

 according to Kohiker's account of their development (/. c, p. 119); 

 and in that case they are the homologues of the root-sheaths and 

 enamel-organs of Hairs and Teeth. 



1 Sei Todd and Bowman, p. 175. 



