THE TEETH. 211 



Each tooth, is a hollow cone ; the central or pulp-cavity con- 

 taining the blood-vessels and nerves. The basal part of the cone 

 consists of bone, the apical part of dentine, capped at the tip by 

 a very thin layer of enamel. The teeth are readily lost, and are 

 replaced by new ones developed below them. 



The teeth do not begin to form until the time of the meta- 

 moi-phosis. Round the border of the jaw, a soUd ridge-like 

 ingrowth of the deeper layer of the epidermis takes place into 

 the underlying connective tissue ; and opposite the edge of this 

 ridge a series of small processes, the dentinal papillee, are formed 

 in the dermis. The^e papillge, which are very rich in cells, 

 grow into the epidermal ridge, which thus forms a cap over each 

 ■of them. The inner lining of each cap, immediately covering 

 the papilla, consists of a single layer of short columnar cells, the 

 enamel cells ; while the rest of the cap, which is three or four 

 ■cells thick, consists of indifferent cells, the outermost layer of 

 which forms a more or less definite capsule. 



Of the hard tissues of the tooth, the thin cap of enamel is 

 formed by calcification of the ends of the enamel cells next to 

 the papilla. The dentine is formed by calcification of the sur- 

 face layer of the papilla itself, the cells of the papilla sending 

 processes into the dentinal substance while it is forming. 



The young tooth now separates from the epithelial ridge, 

 and moves towards the surface ; it lengthens, by formation of 

 bony matter at its base, but does not acquire its definite attach- 

 ment to the bones of the jaw until some time after the comple- 

 tion of the metamorphosis, usually during the autumn of the first 

 year. The dermal papilla persists as the pulp of the tooth. 



The replacing teeth are developed in precisely similar fashion, 

 and from the original epidermal ridge. They lie at first to the 

 inner side of the first row of teeth, but during their development 

 «hift their positions so as to lie directly above these. By further 

 growth downwards, accompanied by absorption of the bony 

 bases of the teeth in use, the new teeth move towards the 

 surface; they often lie for a time partly within the pulp cavities 

 of their predecessors, and, as these latter fall out, speedily grow 

 into their places. 



The vomerine teeth are straighter than those of the margin 

 of the jaw, but are otherwise similar to these, both in structure 

 and in mode of development. 



p 2 



