ENAMEL 55 



epithelium of the mouth, which first becomes thickened in the neighborhood 

 of the maxillae or jaws, now also in the course of formation. This epidermal 

 papilla grows downward into a recess of the imperfectly developed tissue of the 

 embryonic jaw. It forms the primary enamel organ or enamel germ, and 

 its position is indicated by a slight groove in the mucous membrane of the 

 jaw. The next step consists in the elongation and the inclination outward 

 of the deeper part, figure 65, B, /', of the enamel germ, followed by an 

 increased development at certain points corresponding to the situations of 

 the future milk-teeth. The enamel germ becomes divided at its deeper 

 portion, or extended by further growth, into a number of special enamel 

 germs corresponding to each of the milk-teeth, and connected to the com- 

 mon germ by a narrow neck. Each tooth is thus placed in its own special 

 recess in the embryonic jaw, figure 65, c, f '. 



As these changes proceed, there grows up from the underlying tissue 

 into each enamel germ, figure 65, c, p, a distinct vascular papilla, dental 



FIG. 66. Part of Section of Developing Tooth of a Young Rat, showing the Mode of Deposi- 

 tion of the Dentine. Highly magnified, a, Outer layer of fully formed dentine; b, uncalcified 

 matrix with one or two nodules of calcareous matter near the calcified parts; c, odontoblasts send- 

 ing processes into the dentine; d, pulp; e, fusiform or wedge- shape cells found between odonto- 

 blasts; /, stellate cells of pulp in fibrous connective tissue. The section is stained in carmine, which 

 colors the uncalcified matrix but not the calcified part. (E. A. Schafer.) 



papilla, and upon it the enamel germ becomes molded, and presents the 

 appearance of a cap of two layers of epithelium separated by an interval, 

 figure 65, c, I'. While part of the sub-epithelial tissue is elevated to form 

 the dental papillae, the part which bounds the embryonic teeth forms the 

 dental sacs, figure 65, C, s; and the rudiment of the jaw sends up processes 

 forming partitions between the teeth. The papilla, which is really part 

 of the dental sac, is composed of nucleated cells arranged in a meshwork, 

 in the outer layer of which are the columnar cells called odontoblasts. The 

 odontoblasts form the dentine, while the remainder of the papilla forms the 

 pulp. The method of the formation of the dentine from the odontoblasts 

 is said to be as follows: The cells form elongated orocesses at their outer 

 surfaces which are directly converted into the tubules of dentine, figure 66, c, 

 and into the contained fibrils. 



