390 H. E. JORDAN 



the so-called papillary layer is not the same thing in both cases. 

 It is, however, formed in exactly the same way and through the 

 operation of exactly the same factors: pressure upon the outer 

 surface of the enamel organ by impinging capillaries. The 

 papillary layer of the enamel organ is accordingly simply a 

 secondary mechanical modification of the peripheral portion of 

 the enamel organ. In the cat's incisors, where the enamel organ 

 is relatively thick, this modification involves only the outer en- 

 amel epithelium and the immediately adjacent peripheral portion 

 of the stellate reticulum. It never involves the stratum inter- 

 medium, except, in an indirect way, in the later stages of develop- 

 ment when the stellate reticulum has disappeared and the amelo- 

 blasts have practically ceased functioning. In the case of the 

 rat's incisor, where the enamel organ is thin, due to the absence 

 of a stellate reticulum, this same pressure of the capillaries upon 

 the outer enamel epithelium pushes the latter into contact with 

 the stratum intermedium, and in consequence the resulting 

 'papillary layer' involves all of the extra-ameloblastic layers of 

 the enamel germ. Accordingly, contrary to the view of Williams 

 and of Skillen, the papillary layer has no specific significance in 

 amelogenesis distinct from the extra-ameloblastic portion of the 

 enamel organ as a whole. 



c. The stratum mtermedium 



This layer varies considerably in different teeth and in different 

 regions of the same tooth. It is present over those areas where 

 enamel is never formed as well as over the crown of the tooth. 

 Being next the layer of ameloblasts, and in intricate continuity 

 with these cells, as well as with the cells of the stellate reticulum, 

 it of necessity is most closely concerned with the transportation 

 to the ameloblasts of whatever material may be passed from 

 the blood vessels through the stellate reticulum for the support 

 of the metabolic processes of amelogenesis. Since in the incisor 

 teeth' of rodents the papillary layer is composed largely of cells 

 comparable to those of a stratum intermedium, and in fact 

 constitutes almost the whole of the extra-ameloblastic portion of 

 the enamel germ of the crown, William's statement that the cells 



