68 



HISTOLOGY. 



M. 



D.R. 



only at the foramen. Nowhere does the dentine reach the outer surface 

 of the tooth. In the root it is covered by the cement layer which increases 

 in thickness from the neck toward the apex; and in the crown it is enclosed 

 by the broad layer of enamel. The enamel, however, becomes thin toward 

 the neck, where it meets and is sometimes overlapped by the cement. The 

 pulp, dentine, and sement are of mesenchymal origin, the dentine and cement 

 being varieties of bone. The enamel is an ectodermal formation, but so 

 intimately associated with the others that it may be described with them. 



In the human fetus of about two months the ectoderm covering the 

 jaws is continuous with the entoderm lining the mouth and throat, as 

 shown in Fig. 75, and there is nothing to indicate where they join. To- 

 ward the front of the mouth, in either jaw, the epithelium sends a plate-like 

 prolongation into the underlying mesenchyma. This is called the dental 



ridge. There is a continuous ridge parallel 

 with the circumference of each jaw, and that 

 it is entirely ectodermal is known from the 

 study of earlier stages when the oral plate is 

 still present. In the diagram, Fig. 76, at A, a 

 part of the ridge in the lower jaw and of the 

 oral epithelium from which it grows, is repre- 

 sented as free from the surrounding mesen- 

 chyma. The labial side of the ridge is toward 

 the left and the lingual side toward the right. 

 The ridge later produces a series of inverted 

 cup-shaped enlargements along its labial sur- 

 face and these become the enamel organs. 

 There is an enamel organ for each of the ten 

 deciduous or temporary teeth in either jaw. 

 Within the inverted cus the mesenchyma becomes very dense, producing 

 in each a dental papilla from which the pulp and dentine are derived. The 

 enamel organ produces the enamel, and perhaps controls the shape of the 

 tooth. The cement is derived from the surrounding mesenchyma. 



Three stages in the formation of enamel organs and papillae are 

 shown in Fig. 76. The dental groove in C is a transient depression which 

 is relatively unimportant. In D the enamel organs are connected with 

 the dental ridges by slender necks of epithelial tissue which subsequently 

 become severed. At about eleven weeks all the papillae and enamel 

 organs of the deciduous teeth have formed. The permanent teeth develop 

 from similar organs and papillae which arise later; the first molars are 

 indicated at five months, and in embryos of six months (30-40 cms.) 

 all of the permanent front teeth may be found. Their enamel organs 



Md 



FIG. 75. 



Part of a sagittal section of 

 human embryo, to show the 



Basition of the dental ridges, 

 . R.; M., mouth; Md., 

 mandible; My., maxilla.' N., 

 median nasal septum; P., 

 palate. 



