ON THE DEVELOPMENT OF THE TEETH 225' 



with the teeth. Having endeavoured to answer them, I shall inquire 

 with what other organs of the animal the teeth correspond. 



I. The nature of the pulp, the capsule, and the enamel organ, with 

 relation to the mucous membrane from which they are developed. 



The teeth are developed in two ways, which are, however, mere 

 varieties of the same mode in the animal kingdom.^ In the first, 

 which may be typified by the Mackerel and the Frog, the pulp is 

 never free, but from the first is included within the capsule, seeming 

 to sink down as fast as it grows. 



In the other the pulp projects freely at one period above the 

 surface of the mucous membrane, becoming subsequently included 

 within a capsule formed by the involution of the latter : a marked 

 instance of this mode of development occurs in the human subject. 

 The Skate offers a sort of intermediate stage. 



If the thick and opaque, coloured, mucous membrane of the jaw of 

 the Mackerel be torn away, and the alveolar edge of the jaw be then 

 examined with a low power, minute germs will be seen to be imbedded 

 in the substance of the jaw, among the large, fully-formed teeth. One of 

 the smallest of those which I examined is figured at PI. III. [XXIII.]. 

 fig. lO. It was an oval mass, about i-6oth of an inch in long diameter ; 

 its upper part was roofed as it were by the epithelium of the gum ; its 

 sides were constituted by the continuation of the basement membrane 

 of the mucous membrane of the mouth ; within this was a homogene- 

 ous substance, containing numerous oval or rounded nuclei, about 

 I -5000th of an inch in diameter, and continuous with the lowest layer 

 of the epithelium of the mouth. In the centre appeared a large 

 conical mass, nearly as long as the sac, the proper tooth pulp. 

 Pointed above, it widened below, and then gradually contracted again, 

 so as to form an almost hemispherical lower extremity, which was 

 united to the base of the sac by a narrow neck. In the upper part of^ 

 the papilla the proper dental tissues had already begun to make their 

 appearance ; but below, a delicate membrane formed its outer 

 boundary, and this passed directly into the basement membrane of 

 the sac. 



It is clear then, that in this case the papilla is wholly a process of 

 the derm (or that which in a mucous membrane corresponds to it) 

 outwards, while the sac is a process inwards of the same structure ; 

 and that the homogeneous substance, with its imbedded nuclei between 

 the two, corresponds with the epidermis or epithelium. 



1 For the purposes of the present examination I have taken the Skate, the Mackerel, the 

 Frog, the Calf, and Man, as accessible specimens of each of the great divisions of animals, 

 possessing teeth. 



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