DEVELOPMENT OF THE TEETH. 



1557 



reason it is termed the dental ledge. It will be seen that the formerly described pri- 

 mary stage of the dental groove is unfounded, since the furrow that does exist is 

 secondary and not directly related to the formation of the teeth, but to the dilTer- 

 entiation of the lips. During the third foetal month the anlages for the entire set of 

 milk-teeth become evident along the dental bar, coincidently, by the eleventh week, 

 the completion of the labial furrozv separating the lip from the original epithelial 

 strand with which the dental ledge alone for a time remains attached. 



The anlages of the milk-teeth are indicated by club-shaped epithelial outgrowths 

 which grow down from the deeper surface of the dental ledge to form the enamel- 



FiG. 1316. 



D 



- ■"" wirffTSy^a? 



Reconstructions of oral ectoblast of human embryos ; only epithelium of lips, mouth, and enamel-organs shown. 

 A, embryo of 2.5 cm. length; /«, oral opening; e, labial epithelium; Id, reverse of labio-dental groove; rfj, dental 

 ledge, i)', embryo of 4 cm.; /a, projection caused by labio-dental groove; rfj, dental ledge. C, embryo of 11.5 cm., 

 or of about fourteen weeks; w', enamel-organ of first molar tooth. Z>, embryo of 18 cm., or of about seventeen 

 weeks ; i- m 1, enamel-organs of second incisor and of first molar teeth. {Drawn from Rose's models.) 



organs and to meet, and later cap, the mesoblastic elevations or dental papilla'. With 

 the rapid growth and expansion of the extremity of the epithelial plug, a differentia- 

 tion of the latter into the typical three-layered enamel-organ takes place, the pro- 

 jecting dental papilla apparently invaginating the overlying epithelial structure. At 

 first connected by a broad band of cells, the attachment of the enamel-organ with the 



