72 



THE CONNECTIVE TISSUES 



[CH. V. 



the imperfectly developed tissue of the embryonic jaw. The down- 

 ward epithelial growth forms the common enamel or dental germ, and 

 its position is indicated by a slight groove in the mucous membrane 



of the jaw. After this there 

 is an increased development 

 at certain points correspond- 

 ing to the situations of the 

 future milk-teeth. The com- 

 mon enamel germ thus be- 

 comes extended by further 

 growth into a number of 

 special enamel germs (fig. 

 98, B,) corresponding to each 

 of the milk-teeth, and con- 

 nected to the common germ 

 by a narrow neck (/). Each 

 tooth is thus placed in its 

 own special recess in the 

 embryonic jaw. 



As these changes proceed, 

 there grows up from the 

 underlying connective tissue 

 into each enamel germ (fig. 

 98, c, p), a distinct vascular 

 papilla (dental papilla), and 

 upon it the enamel germ be- 

 comes moulded, and presents 

 the appearance of a cap of 

 two layers of epithelium 

 separated by an interval (fig. 

 98, c,/). Whilst part of the 

 subepithelial tissue is ele- 

 vated to form the dental 

 papilla, the part which bounds 

 the embryonic teeth forms 

 the dental sac (fig. 98, c, s) ; 

 and the rudiment of the 

 jaw sends up processes form- 

 ing partitions between the 

 teeth. In this way small 



chambers are produced in which the dental sacs are contained, 

 and thus the sockets of the teeth are formed. The papilla is com- 

 posed of nucleated cells arranged in a mesh-work of connective 

 tissue, the outer or peripheral part being covered with a layer of 

 columnar nucleated cells called odontoblasts. 



FIG. 98. Section of the upper jaw of a foetal sheep. 

 A. 1, common enamel germ dipping down into the 

 mucous membrane ; 2, palatine process of jaw ; 

 3, Rete Malpighi. B. Section similar to A, but 

 passing through one of the special enamel germs 

 here becoming flask-shaped ; c, d, epithelium of 

 mouth; /, neck ; /, body of special enamel germ. 

 C. A later stage ; c, outline of epithelium of gum ; 

 /, neck of enamel germ ; /, enamel organ ; p, papilla ; 

 s, dental sac forming ; / p, the enamel germ of per- 

 manent tooth ; m, bone of jaw ; v, vessels cut across. 

 (Waldeyer and Kolliker.) 



