DEVELOPMENT. 



15 



in the rest of the class. In these primitive papillae there can be very- 

 early distinguished a cavity containing fluid, and a dense membrane, 

 fmemhrana propria pulpij surrounding the cavity, and itself covered by 

 the thin external buccal mucous membrane, which gradually becomes 

 more and more attenuated as the papilla increases in size. In some 

 fishes, as the sharks and rays, the dental papillae do not sink into the 

 substance of the vascular membrane from which they grow, but be- 

 come buried in depressions of an opposite fold of the same mem- 

 brane ; these depressions enlarging with the growth of the papillee, 

 and forming the cavities or capsules in which the development of the 

 tooth is completed. They differ from the capsules of the matrix of 

 the mammiferous tooth in having no organic connexion with the 

 pulp, and no attachment to its base : the teeth when fully formed are 

 gradually withdrawn from the above described extraneous capsules, to 

 take their place and assume the erect position on the alveolar border 

 of the jaws. 



Here, therefore, is represented on a large and, as it were, persis- 

 tent scale, the first and transitory papillary stage of the develop- 

 ment of the mammalian teeth ; and the simple crescentic cartilagi- 

 nous maxillary plate, with the mucous groove behind it containing 

 the germinal papillae of the teeth, offers in the shark a magnified re- 

 presentation of the earliest condition of the jaws and teeth in the 

 human euibryo. 



In many fishes, as the lophius and pike, the dental papillae become 

 buried in the membrane from which they arise, and the surface to 

 which their basis is attached becomes the bottom of a closed sac. 

 But this sac is never lodged in the substance of the jaw, the develop- 

 ment of the tooth being completed in the tissue of the thick and soft 

 gum or mucous membrane from which the papillae were originally 

 developed : hence teeth in various stages of growth are frequently 

 brought away with that membrane when it is reflected from the jaw- 

 bone. The ultimate fixation of the teeth, so formed, is effected by the 

 development of ligamentous fibres in the submucous tissue between 

 the jaw and the base of the tooth ; which fibres become the medium 



many teeth, and equally militates against the theory of formation by transudation of layers 

 being applied, at least, to thp ganoid scales. 



