388 H. E. JORDAN 



DISCUSSION 



a. General 



The fundamental cause of the structural differences in the 

 enamel organ of the incisor teeth of the white rat and of rodents 

 in general (with the exception of the suborder, Lagomorpha, 

 including rabbits and hares) as compared with those of other 

 mammals, inheres in the fact that the rodents' incisors grow 

 persistently throughout life. They lack a genuine root, having 

 a persistent pulp; and they have no deciduous predecessors. 

 The restriction of the enamel organ of the rat's incisor to the 

 outer surface of these teeth is correlated with a restriction of 

 definitive enamel to this surface. The relatively thinner charac- 

 ter of the rat's enamel organ, in which a stellate reticulum is 

 practically lacking, may be correlated with the relatively more 

 restricted and more compact dental sac, as compared with condi- 

 tions in the cat, effecting in consequence greater pressure upon 

 the enamel organ, to the suppression of development of a stellate 

 reticulum. The absence of a stellate reticulum may perhaps 

 be the result in part also of the greater attenuation of the 

 enamel germ in these incisors, because spread over a larger sur- 

 face, and more rapidly, in the case of these relatively long and 

 rapidly growing teeth. The better developed character of the 

 'papillary layer' follows from the thinner condition of the 

 enamel germ, and results from pressure on the part of imping- 

 ing capillaries scattered over its surface. 



The several differential characteristics of the enamel organ 

 of the incisor of the white rat and the kitten throw much light 

 upon the question of the significance of the enamel organ as a 

 whole, and of its constituent layers: ameloblastic layer, stratum 

 intermedium, stellate reticulum, and the outer enamel epithelium. 



The function of the ameloblasts need not be discussed. It 

 may be accepted as proved that they are the essential elements 

 in the elaboration of enamel. As to how in detail the ameloblasts 

 produce enamel prisms need not concern us here. We are 

 interested chiefly in the question as to how the other layers of 

 the enamel germ cooperate in this process. 



