THE STOMOD/KUM. 



777 



and branchial arches ; and very possibly the teeth on some other parts of 

 the mouth are developed in a true hypoblastic region. 



The teeth are formed from two distinct organs, viz. an epithelial cap and 

 a connective tissue papilla. 



The general mode of development, as has been more especially shewn 

 by the extended researches of Tomes, is practically the same for all Verte- 

 brata, and it will be convenient to describe it as it takes place in Mam- 

 malia. 



Along the line where the teeth are about to develop, there is formed 

 an epithelial ridge projecting into the subjacent connective tissue, and 

 derived from the innermost columnar layer of the oral epithelium. At the 

 points where a tooth is about to be formed this ridge undergoes special 

 changes. It becomes in the first place somewhat thickened by the develop- 

 ment of a number of rounded cells in its interior ; so that it becomes 

 constituted of (i) an external layer of columnar cells, and (2) a central core 

 of rounded cells ; both of an epithelial nature. In the second place the 

 organ gradually assumes a dome-shaped form (fig. 428, e), and covers over a 

 papilla of the subepithelial connective tissue (p] which has in the meantime 

 been developed. 



From the above epithelial structure, which may be called the enamel 

 organ, and from the papilla it covers, which 

 maybe spoken of as the dental papilla, 

 the whole tooth is developed. After these 

 parts have become established there is formed 

 round the rudiment of each tooth a special 

 connective tissue capsule ; known as the 

 dental capsule. 



Before the dental capsule has become 

 definitely formed the enamel organ and the 

 dental papilla undergo important changes. 

 The rounded epithelial cells forming the core 

 of the enamel organ undergo a peculiar trans- 

 formation into a tissue closely resembling 

 ordinary embryonic connective tissue, while 

 at the same time the epithelium adjoining 

 the dental papilla and covering the inner 

 surface of the enamel organ, acquires a some- 

 what different structure to the epithelium 

 on the outer side of the organ. Its cells 

 become very markedly columnar, and form 

 a very regular cylindrical epithelium. This 

 layer alone is concerned in forming the 

 enamel. The cells of the outer epithelial 

 layer of the enamel organ become somewhat 

 flattened, and the surface of the layer is raised into a series of short papilla? 

 which project into the highly vascular tissue of the dental sheath. Between 



FIG. 428. DIAGRAM SHEW- 

 ING THE DEVELOPMENT OF THE 

 TEETH. (From Gegenbaur.) 



p. dental papilla ; e. enamel 

 organ. 



