7 6 



SEVENTH LECTURE. 



been described, is connected a very complicated history of 

 their origin. We here mention only the chief points. 



That the teeth are formed in the maxillary bones, that in 

 the infant the eruption first takes place after months and 

 years, that the first teeth are for the greater part replaced by 

 permanent ones, is known to all. 



Two of the three germinal plates participate in the produc- 

 tion of our structures, the corneous layer and the middle ger- 

 minal layer. The former produces the enamel, the latter the 

 pulp, dentine and cement. 



On the free borders of 

 the embryonic jaw ap- 

 pears at first a mound- 

 like thickening of the 



pavement 

 (Fig. 70, a). 



epithelium 

 It presses 



Fig. 70. — Tooth formation of a 

 hog's embryo ; a, epithelial 

 mound ; b, younger cell layer ; c, 

 the lowermost ; e, enamel organ ; 

 f, tooth germ ; g, inner, and h, 

 outer layer of the progressing 

 tooth sac. 



Fig. 71. — Tooth sac of an older human embryo, partly 

 diagramatic ; a, connective-tissue parietes nf the 

 tooth sac. with the outer layer at a 1 and the inner at 

 a' 1 ; b, enamel organ, with its external, c, and inferior 

 cells, d ; e, dentine cells ; f. dentine with the capillary 

 vessels,^; i, transition of the connective tissue of the 

 parietes into the tissue of the dentine germ. 



downwards into the soft substance of the maxillary tissue as a 

 vertical elongated ridge. The former has been named the 

 tooth papilla, the latter the enamel germ. 



From place to place, springing up from the depths of the 

 tissue of the jaw, convex papillar structures, the so-called 

 tooth germs (/), grow towards the enamel germ. Here and 

 there, increasing in diameter, they press in the under surface 



