DEVELOPMENT OF THE TEETH. 153 



(common enamel-germ, fig. 178, A). At regular intervals there is yet a 

 further thickening and growth from the common enamel-germ into the 

 tissue of the mucous membrane, each of these special rudiments swelling 

 out below into a flask-shaped mass of cells, the special enamel-germ, 

 fig. 178, B). A vascular papilla grows up from the corium into the 

 bottom of the special enamel-germ (fig. 178, C, D) ; this papilla has 

 the shape of the crown of the future tooth. Each special enamel- 

 germ, with its included papilla, presently becomes cut off from the 

 epithelium of the mouth, and surrounded by a vascular membrane 

 the dental sac. The papilla becomes transformed into the dentine of 

 the future tooth, and the enamel is deposited upon its surface by the 

 epithelial-cells of the enamel-germ. The root of the tooth, with its 

 covering of cement, is formed at a later period, when the tooth is 

 beginning to grow up through the gum, by a gradual elongation of the 

 base of the papilla. 



Previously to the deposition of the enamel, the enamel-germ under- 

 goes a peculiar transformation of its previously-rounded epithelium- 

 cells into three layers of modified cells. One of these is a layer of 

 columnar cells (fig. 179, d\ which immediately covers the surface 

 of the dentine. These columnar cells form the enamel-prisms either 

 by a deposition of calcareous salts external to them, or by a direct 

 calcification of their protoplasm. The cells next to the dental sac 

 form a single layer of cubical epithelium (e), nearly all the other cells 

 of the enamel-germ become transformed into branching corpuscles (c) 

 communicating by their processes, and thus forming a continuous net- 

 work. The enamel-germ, after it is thus modified, is known as the 

 enamel-organ. 



The dentine of the tooth is formed by calcification of the surface of 

 the papilla. At this surface there is a well-marked layer of odonto- 

 blasts (fig. 180), and these produce a layer of dentinal matrix which 

 forms a sort of cap to the papilla, and which soon becomes calcified by 

 the deposition of globules of calcareous matter. Processes of the odonto- 

 blasts remain in the dentine as it is forming, and thus the dentinal 

 tubules are produced. Subsequently other layers of dentine are formed 

 within the first by a repetition of the same process, and in this way the 

 papilla gradually becomes calcified. A part, however, remains un- 

 altered in the centre of the tooth, and with its covering of odontoblasts 

 forms the pulp. 



The ten milk-teeth are formed in each jaw in this manner. These, 

 however, become lost within a few years after birth, and are replaced 

 by permanent teeth in much the same way that a new succession of 

 hairs occurs. A small outgrowth takes place at an early period from 



