56 CELL DIFFERENTIATION AND THE ELEMENTARY TISSUES 



The fibers are thus marked by transverse lines. They are mostly solid, 

 but some of them may contain a very minute canal. 



The enamel prisms are connected together by a trace of hyaline cement 

 substance. 



Development. The first step in the development of the teeth consists 

 in a downward growth, figure 65, A, i, from the deeper layer of stratified 

 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 



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

 of Deposition 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 calcined parts; 



c, odontoblasts sending processes into the dentine; d, pulp; e, fusiform or wedge-shape cells 

 found between odontoblasts; /, 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.) 



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

 creased 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 corre- 

 sponding to each of the milk-teeth, and connected to the common germ by a 

 narrow neck. Each tooth is thus placed in its own special recess in the 

 embryonic jaw, figure 65, c, /'. 



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

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

 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, /'. While part of the subepithelial 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 



