560 THE HUMAN EMBRYO. 



the enamel organ becomes closely fitted, like a cap, over the 

 surface of the papilla, which acquires the form of the crown of 

 the future tooth. 



From the dental papilla the main substance of the tooth, or 

 dentine, is formed in the following manner. On the surface of 

 the papilla next the enamel organ a layer of special cells, the 

 odontoblasts, appear. These form, by excretion on their outer 

 surfaces, a dense matrix in which fine filamentous processes of 

 the odontoblasts are embedded ; by .calcification of the matrix 

 the dentinal substance is formed, the dentinal tubules being the 

 narrow channels in the matrix occupied by the processes of the 

 odontoblasts. The first formed part of the dentine is the outer- 

 most layer of the crown of the tooth, and this layer thickens by 

 further formation of dentine on its inner surface, the odonto- 

 blasts gradually withdrawing further and further from the 

 surface, as the dentine increases in thickness. 



The enamel is formed from the layer of epithelial cells of the 

 enamel organ which lies in immediate contact with the dental 

 papilla. This layer consists of closely set, columnar or prismatic 

 cells, and it is by direct calcification of these cells that the 

 enamel is produced. The rest of the enamel organ is merely 

 nutritive in function, and does not give rise directly to any 

 part of the tooth. 



The crown is thus the first part of the tooth to be formed. 

 After it is completed, the tooth increases in length by the 

 further formation of dentine round the lower part of the papilla. 

 The aperture at the base of the tooth is at first a widely open 

 one ; but, as the tooth approaches its full size, the aperture 

 becomes gradually narrowed to form the root or fang of the 

 tooth. In the case of the grinding teeth, the aperture becomes 

 divided by bridges of dentine into two or three separate openings, 

 and by elongation of the margins of these openings the double or 

 triple fangs of the adult tooth are produced. At the apex of each 

 fang a minute hole remains, through which the blood-vessels and 

 nerves gain admittance to the pulp of the tooth, which latter is the 

 part of the dental papilla that remains encapsnled in the middle 

 of the tooth after its completion. 



The cement, or outermost layer of the fully formed tooth, is a 

 bony deposit developed from the connective-tissue sheath which 

 surrounds it. 



