130 



THE ESSENTIALS OF HISTOLOGY 



beginning to grow up through the gum, by a gradual elongation of the 

 base of the papilla. 



Previously to the deposition of the enamel, the enamel-germ under- 

 goes a peculiar transformation of its previously rounded epithelium- 



FIG. 156. A SECTION THROUGH THE 



ENAMEL-ORGAN AND DENTAL SAC FROM 

 THE TOOTH OF A CHILD AT BIRTH. 



(250 diameters.) 



a, outer dense layer of the dental sac; 6, inner 

 looser texture of the same with capillary blood- 

 vessels and a somewhat denser layer towards 

 the enamel organ ; c, spongy substance ; d, inner 

 cells ; and e, outer cellular layer of the enamel- 

 organ. 



FIG. 157. PART OF SECTION OF DEVELOP- 

 ING TOOTH OF YOUNG RAT, SHOWING 

 THE MODE OF DEPOSITION OF THE DEN- 

 TINE. (Highly magnified.) 



a, outer layer of fully calcined dentine ; 6, un- 

 calcified matrix, with a few nodules of calca- 

 reous matter ; c, odontoblasts with processes 

 extending into the dentine ; d, pulp. The sec- 

 tion is stained with carmine, which colours 

 the uncalcifled matrix, but not the calcified 

 part. 



cells into three layers of modified cells. One of these is a layer of 

 columnar cells (fig. 156, d}, which immediately covers the surface 

 of the dentine. These columnar cells form the enamel-prisms either 

 by a deposition of calcareous salts external to them, or by a direct 

 calcification of their protoplasm. The cells next to the dental sac 

 form a single layer of cubical epithelium (e), all the other cells of the 



