DENTINE.— ENAMEL.— LENS TISSUE. 



77 



of the enamel germ, and thus give this locally the form of a 

 cap or bell. The latter is called the enamel organ (e). 



Leaving the intermediate forms aside, let us pass at a 

 bound to a later period. Here (Fig. 71) the enamel organ 

 (I?) has long since become separated by constriction from its 

 point of origin, the epithelium of the jaw, and also thrown 

 off the lateral bridge connecting it with the ridge of the enamel 

 germ. It is covered on the upper convex and inferior concave 

 surfaces with cylindrical epithelial cells (c, d). In the interior 

 (/?) we find gelatinous tissue (Fig. 22). Below (Fig. 71, f) 

 we perceive the thick tooth germ, the progressing tooth crown. 

 Both are enclosed within a connective-tissue capsule (a), the 

 so-called tooth sac, with external (a 1 ) and internal (a 2 ) layers. 

 The sac and tooth germ finally become continuous with each 

 other below. 



The tooth germ bears on its surface the layer of odonto- 

 blasts [e). From them is produced the first thin cortical layer 

 of the dentine. Layer on layer are subsequently formed 

 over the longitudinally growing tooth germ. By this growth 

 it finally obtains the neck and fang ; 

 its soft, vascular tissue remains more 

 and more retarded in its further 

 development, and becomes the pulp. 

 From the epithelium at the concave 

 surface of the enamel organ, occurs 

 the formation of the enamel prisms 

 (below d), whether these represent 

 calcified portions of the cell body or 

 secreted cell substances. The tooth, 

 growing up, kills the enamel organ 

 at last, and makes its eruption. Its 

 cement may originate from the lower 

 portion of the tooth sac. This per- 

 sists, for the most part, as the peri- 

 osteum of the alveolus. 



For the permanent teeth, a secondary enamel germ appears 

 to branch off from the original one at a very early period. 



Fig. 72. — Crystalline lens ; a, cap- 

 sule ; b, epithelium of the anterior 

 half; c, lens fihres, with the anterior, 

 d, and posterior ends, e ; _/, nuclear 

 zone. 



