DEVELOPMENT OF THE TEETH. 491 



only of cement, dentine, and probably also always of cuticle, 

 requires revision. According to the statements of Owen (25), 

 these neither possess an enamel organ, nor form a closed dental 

 sacculus.* We possess no accurate information of the relations 

 of the oral epithelium. Probably there is here, as Leydig (36) 

 describes in several species, as, for example, in the Anguis 

 fragilis, a thin covering to the freely projecting dental papilla, 

 which subsequently becomes the horny cuticle. In accordance 

 with a more recent investigation of Leydig (62), the crowns of 

 the teeth, which, however, have no investing enamel, originate 

 in Salamandra maculosa, in several dental sacculi which lie at 

 the bottom of the " epithelium of the jaw." The fangs are de- 

 veloped from the subjacent connective tissue. Leydig considers 

 the substance of the dental crowns to be a cuticular formation. 



The simple horny teeth do not differ in their formation from the 

 ordinary papillae of the oral mucous membrane possessing a strong 

 horny investment. Nothing is at present known of the mode of de- 

 velopment of the more compound forms occurring in Ornithorhyncus 

 and others. 



Accurate knowledge of the dental tissues, and of their de- 

 velopment, commences with the works of Purkyne' and his 

 scholars, Frankel (13) and Raschkow (14). Leeuwenhoek (2) 

 had indeed previously seen the dental canaliculi, and, like J. 

 Hunter (4), had recognised the cement as a distinct substance, 

 the discovery of which is ordinarily attributed to Blake (5) 

 and Tenon (6) ; still it is only from the time of Purkyne that 

 the knowledge of this subject has become common property. 

 The enamel fibres have been described by many from the time 

 of Malpighi. Retzius (19) and Hannover (39) gave the most 

 accurate description of the structure of the dentine and enamel, 

 especially with regard to the various lines and markings upon 

 and in them, and the course of the canaliculi and enamel fibres. 

 Nasmyth (22) and Erdl (27) first described ' the cuticle, and 

 Czermak (33) the interglobular substance. We are indebted to 

 E. Neumann (48) for the demonstration of the dental sheaths, 



* Owen, moreover, claims enamel for many animals, in which it does to 

 exist, as, for instance, the frog. 



M M 



