7 8 



THE CONNECTIVE TISSUES. 



[CH. V. 



becomes thickened in the neighbourhood of the maxillae or jaws 

 now in the course of formation. This process passes downward 

 into a recess of the imperfectly developed tissue of the embryonic 

 jaw. The downward 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. The next step in the process 

 consists in the elongation downward of the enamel groove and 

 of the enamel germ and the inclination outward of the deeper 

 part (fig. 98, B, /'), which is now inclined at an angle with the 

 upper portion or neck (/), and has become bulbous. After this 

 there is an increased development at certain points corresponding 

 to the situations of the future milk-teeth. The common enamel 

 germ thus becomes extended by further growth into a number 



of special enamel germs 

 corresponding to each of 

 the above-mentioned milk- 

 teeth, and connected to the 

 common germ by a narrow 

 neck. Each tooth is thus 

 placed in its own special 

 recess in the embryonic jaw 



(fig. 9 8, B, ff'). 



As these changes pro- 

 ceed, there grows up from 

 the underlying connective 

 tissue into each enamel 

 germ (fig. 98, c, p), a dis- 

 tinct vascular papilla (den- 

 tal papilla), and upon it 

 the enamel germ becomes 

 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 elevated 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 forming 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 composed of nucleated cells arranged in a mesh- 

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

 columnar nucleated cells called odontoblasts. The odontoblasts 

 form the dentine, while the remainder of the papilla forms the 

 tooth-pulp. 



Fig. 99. Part of section of developing tooth of a 

 young rat, showing the mode of deposition of 

 the dentine. Highly magnified, a, outer layer 

 of fully formed dentine ; b, uncalcified matrix 

 with one or two nodules of calcareous matter 

 near the calcified parts ; c, odontoblaste send- 

 ing processes into the dentine ; d, pulp ; 

 e, fusiform or wedge-shape cells found between 

 odontoblasts ; /, stellate cells of pulp in fibrous 

 connective tissue. The section is stained with 

 carmine, which colours the uncalcified matrix 

 but not the calcified part. (E. A. Schafer.) 



