HISTOLOGY OF ENAMEL ORGAN 387 



surface. Transverse sections of the head and the upper jaw at 

 the level of the dental papilla will therefore show an approxi- 

 mately longitudinal (frontal) section of the radicular portion of 

 this tooth, the enamel organ appearing to cap the coronal sur- 

 face (fig. 3) . The tooth as seen at this point appears to be growing 

 towards the top of the head. Sections farther forward will 

 yield approximately transverse sections (like that of fig. 4), 

 while sections towards the tip of the jaw will again give approxi- 

 mately longitudinal sections of these teeth. The enamel organ 

 is accordingly not restricted to the crown of the tooth, as sections 

 through the radicular area would lead one to infer, but actually 

 to the upper (outer) surface of the tooth. The enamel organ is 

 in fact continued over the coronal apex, but posteriorly its amelo- 

 blasts early undergo regressive changes essentially like those 

 described by Addison and Appleton ('21) for the molar teeth 

 of this same species. Accordingly, enamel is not formed over 

 the coronal apex nor along the inner surface of these incisor 

 teeth. The absence of enamel organ along the inner surface of 

 the upper incisors, and the resulting intimate relation between 

 the forming dentin and the mesenchyme of the dental sac may 

 be correlated with the later continuity along the ventral border 

 basally of the teeth with the alveolar bone. Likewise, the 

 curved manner in which the enamel germ invades the jaw and 

 is met obliquely by the dental papilla, the latter invaginating 

 the former in such manner as to remain bare of enamel organ along 

 the inner surface, is correlated with the future greatly curved 

 form of these teeth and their lack of enamel on the posterior 

 (inner) surface. These peculiar incisor teeth of rodents could be 

 homologized with the more usual form of mammalian teeth by 

 interpreting the outer enamel-covered surface as a greatly elonga- 

 ted crown, the actual chisel-edged and enamel-free crown and 

 the dentin-covered inner surface constituting the subcervical 

 portion of the tooth. 



