Chap, xxi.i THE TEETH. 175 



a petrifaction of this increment, according to others 

 by a secretion by the cell of the dentinal matrix. 

 Waldeyer, Tomes, and others, consider the odontoblasts 

 proper concerned in the production both of the dentinal 

 matrix and dentinal fibres. The odontoblasts proper 

 and the spindle-shaped cells are continuous with the 

 branched cells of the pulp matrix. 



223. Development of teeth. The first rudi- 

 ment of a tooth in the embryo appears as early as the 

 second month. It is a solid cylindrical prolongation of 

 the stratified epithelium of the surface into the depth 

 of the embryonic mucous membrane. Along the 

 border of the jaws the epithelium appears thickened, 

 and the subjacent mucous membrane forms there a 

 depression the primitive dental groove. Into this 

 groove the solid cylindrical prolongation of the surface 

 epithelium takes place. This prolongation is the 

 rudiment of the enamel organ. While continuing 

 to grow towards the depth, it soon broadens at its 

 deepest part, and the surrounding vascular mucous 

 membrane condenses at the bottom of the prolongation 

 as the rudiment of the tooth papilla. While the distal 

 part of the enamel organ continues to grow towards 

 the depth, it gradually embraces the tooth papilla in 

 the shape of a cap the enamel cap. During this 

 time the connection between the surface epithelium 

 and the enamel cap becomes greatly thinned out and 

 pushed on one side, owing to the growth of the enamel 

 cap and papilla taking place chiefly to one side of the 

 original dental groove. 



234. The enamel cap (Fig. 105) is composed of 

 three strata an inner, middle, and outer stratum. 

 The inner stratum is a layer of beautiful columnar 

 epithelial cells, the enamel cells; they were ori- 

 ginally continuous with the deep layer or the 

 columnar cells of the surface epithelium. The middle 

 Stratum is the thickest and is of great transparency, 



