THE DIGESTIVE SYSTEM 243 



Development. — The enamel of the teeth is of ectodermic origin, the re- 

 mainder of mesodermic. The earliest indication of tooth formation occurs about 

 the seventh week of intra-uterine hfe (embryos 12 to 15 mm.). It consists in a 

 dipping down of the epithelium covering the edge of the jaw into the underlying 

 connective tissue (mesoderm) where it forms the dental shelf, or common denial 

 germ. Soon after the formation of the dental shelf, a groove appears along the 

 margin of the jaw where the ingrowth of epithelium occurred. This is known as 

 the dental groove. The epithelium of the dental shelf is at first of uniform 

 thickness. Soon, however, at intervals along the outer side of the dental 

 shelf, the cells of the shelf undergo proliferation and form thickenings, ten in 



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V 







Fig. 147. — Developing Tooth from Three-and-one-half-months' Human Embryo. 

 X65. (Szymonowicz.) a, Epithelium of gums; b, neck of enamel organ; c, dental 

 germ of permanent tooth; d, bone of lower jaw; e, dental papilla;/, inner enamel cells; 

 g, enamel pulp; h, outer enamel cells. 



the upper and ten in the lower jaw, each one corresponding to the position of a 

 future milk tooth. These are known as special dental germs, and remain for 

 some time connected with one another and with the surface epithelium by 

 means of the rest of the dental ridge. 



Into the side of each special dental germ there occurs about the end of the 

 third month (embn,-os of about 40 mm.) an invagination of the underlying con- 

 nective tissue. In the upper jaw the invagination takes place on the upper and 

 inner side, in the lower jaw on the lower and inner side, of each dental germ. 

 Each invagination forms a dental papilla (Fig. 145), over which the tissue of the 



