TEETH. 



6 9 



appear on the labial side of the deep portion of the dental ridge, as shown 

 in Fig. 77, but they are on the inner side of the deciduous teeth. The 

 portion of the dental ridge which is not included in the enamel organs 



Epithelium of the margin 

 of the jaw 



Enamel 

 organs. 



Dental 

 groove. 



Dental ridge 



Necks of enamel organs. 

 D 



FIG. 76. DIAGRAMS SHOWING THE EARLY DEVELOPMENT OF THREE TEETH, ONE OF 



WHICH IS SHOWN IN VERTICAL SECTION. 



k, Free border of the dental ridge. 



sends irregular projections into the mesenchyma and becomes perforated 

 and detached from the oral epithelium. Its remnants found in the gums 

 at birth have been mistaken for glands. A 

 portion of the ridge extends beyond the necks 

 of the enamel organs for the permanent teeth, 

 and this has been said to indicate the possibility 

 of a third set of teeth, a possibility never real- 

 ized in mammals. The second and third 

 molars are formed from a dorsal or backward 

 extension of the dental ridge free from the oral 

 epithelium. The second molars appear in a 

 child of six months, and the third or late molars 

 (wisdom teeth) at five years. The latter are not 

 at the extremity of the dental ridge but are on 

 the labial side of it, so that there is a theoreti- 

 cal possibility of fourth molars. 



O.E. 



D.R. 



E.G. 



FIG. 77. TEETH FROM A HUMAN 

 FETUS OF 30 CMS. (Modified, 

 from Rose.) 



E. and E. O., Eramel organs 

 of a deciduous and of a perma- 

 nent tooth respectively; D. 

 R., dental ridge; O. E., 

 oral epithelium; P., papilla. 



ENAMEL ORGANS AND ENAMEL. 



The enamel organ is at first a mass of undifferentiated epithelial cells, 

 but soon it becomes divisible into three parts as shown in Fig. 78. The 

 inner enamel cells are applied to the dense mesenchymal papilla; the 

 outer enamel cells, continuous at the rim of the cup with the inner cells, 

 are toward the loose mesenchyma; and the enamel pulp fills the space 

 between the outer and inner layers. The outer enamel cells form a single 

 layer of cuboidal cells, with which some flattened cells of the enamel pulp 

 are in close contact. In later stages the layer appears as a feltwork of 



