DEVELOPMENT OF THE TEETH. 479 



or epithelial structures (and are therefore analogous to the enamel), 

 whilst in the lower Vertebrata they are chiefly composed of peculiarly 

 modified and ossified connective tissue ; in the higher classes of 

 animals, which present the most complicated form of dentinal struc- 

 tures, an epithelial structure (the enamel) is again included in their 

 structure. 



DEVELOPMENT OF THE TEETH. The genesis of the teeth 

 in the human embryo commences, according to the observations 

 of Robin and Magitot (46), at about the fiftieth to the sixty- 

 fifth day. The margins of the jaw at the beginning of the 

 third month form a slightly raised rounded ridge, the " maxil- 

 lary ridge" which is most prominent in the lower jaw, and 

 consists of a thickening of the embryonic connective tissue 

 and epithelium of the mucous membrane of the mouth. This 

 epithelium, with its vascular substratum resembling mucous 

 tissue, constitutes therefore the matrix of the several constitu- 

 ents of the teeth, the epithelium forming the enamel, and the 

 mucous tissue the dentine and cement. 



The " enamel organ " is formed by a peculiar structure re- 

 sulting from the growth and multiplication of the epithelial cells, 

 which dip down into the mucous tissue. In a direction con- 

 trary to this there is then developed a papilliform process of 

 the mucous tissue, the origin of the pulp and of the dentine. 

 The two parts together constitute the rudiment of the tooth. 

 When at a later period the connection of the enamel organ 

 with the oral epithelium is interrupted, the rudiment of the 

 tooth is enclosed in the alveolar border of the jaw on all 

 sides, as in a capsule, by the sub-epithelial connective tissue. 

 That portion of the connective tissue which immediately in- 

 vests the rudiment of the tooth is usually termed the " dental 

 sac" and at a later period forms the cement.* 



ENAMEL ORGAN AND ENAMEL. Near the end of the second 

 month of foetal life the margin of the jaw exhibits a slight 



* Kolliker (58) calls the entire rudiment of the tooth enamel organ, 

 papilla dentis, and the connective tissue investment of both, " dental sac- 

 culus," and distinguishes the latter again as " proper dental sacculus," a 

 nomenclature which has little to recommend it. 



