STRUCTURE OF THE ELEMENTARY TISSUES. 



59 



ward epithelial growth forms the primary enamel or nan or enamel 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 down- 

 ward of the enamel groove and of the enamel germ and the inclination 

 outward of the deeper part (Fig. 63, 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, and the enamel germ, or com- 

 mon enamel germ, as it may be called, becomes divided at its deeper por- 

 tion, or extended by further growth, into a number of special enamel 

 germs corresponding to each of the above-mentioned milk teeth, and con- 

 nected to the common germ by a narrow neck, each tooth being placed 

 in its own special recess in the embryonic jaw (Fig. 63, B,//'). 



As these changes proceed, there grows up from the underlying tissue 

 into each enamel germ (Fig. 63, c, p), a distinct vascular papilla (dental 

 papilla), and upon it the enamel germ 

 becomes moulded and presents the ap- 

 pearance of a cap of two layers of epi- 

 thelium separated by an interval (Fig. 

 63, c, /'). Whilst part of the sub- 

 epithelial tissue is elevated to form the 

 dental papilla?, the part which bounds 

 the embryonic teeth forms the dental 

 sacs (Fig. 63, c, s); and the rudiment 

 of the jaw, at first a bony gutter in 

 w r hich the teeth germs lie, 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, which is really part of the dental sac, if 

 one thinks of this as the whole of the sub-epithelial tissue surrounding the 

 enamel organ and interposed between the enamel germ and the develop- 

 ing bony jaw, is composed of nucleated cells arranged in a meshwork, the 

 outer or peripheral part being covered with a layer of columnar nucleated 

 cells called odontoUasts. The odontoblasts form the dentine, while the 

 remainder of the papilla forms the tooth-pulp. The method of the for- 

 mation of the dentine from the odontoblasts is as follows: The cells elon- 

 gate at their outer part, and these processes are directly converted into 

 the tubules of dentine (Fig. 64). The continued formation of dentine 

 proceeds by the elongation of the odontoblasts, and their subsequent con- 

 version by a process of calcification into dentine tubules. The most 

 recently formed tubules are not immediately calcified. The dentine fibres 

 contained in the tubules are said to be formed from processes of the 



FIG. 64. Part of section of developing tooth 

 of a young rat. showing the mode of deposi- 

 tion of the dentine. Highly magnified, a, 

 outer layer of fully formed dentine : ft. uncal- 

 cified matrix with one or two nodules of cal- 

 careous matter near the calcified parts; c, 

 odontoblasts sending processes into the den- 

 tine; d, pulp. The section is stained in car- 

 mine, which colors the uncalcified matrix but 

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



