DIGESTION. 



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. 171, B, f), 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 (fommon enamel germ, as it may be called, becomes divided at 

 its deeper portion, or 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 

 being placed in its own special recess in the embryonic jaw (Fig. 171, B, 



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

 into each enamel germ (Fig. 171, c, p), a distinct vascular papilla (den- 



Fia. 170. Enamel fibres. A, fragments and single fibres of the enamel, isolated by the action 

 of hydrochloric acid. B, surface of a small fragment of enamel, showing the hexagonal ends of 

 the fibres. X 350. (Kolliker. 



tal papilla), and upon it the enamel germ becomes moulded, and pre- 

 sents the appearance of a cap of two layers of epithelium separated by an 

 interval (Fig. 171, c, f). Whilst part of the sub-epithelial tissue is ele- 

 vated to form the dental papillae, the part which bounds the embryonic 

 teeth forms the dental sacs (Fig. 171, c, s) ; and the rudiment of the 

 jaw, at first a bony gutter in which 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 surround- 

 ing the enamel organ and interposed between the enamel germ and the 

 developing bony jaw), is composed of nucleated cells arranged in a mesh- 

 work, the outer or peripheral j>art being covered with a layer of colum- 



