HISTOLOGY OF ENAMEL ORGAN 393 



other side of which the odontoblasts deposit dentin. Since the 

 ameloblasts become differentiated and organized at any point 

 before the odontoblasts, it might be assumed that the latter 

 differentiate under the influence of a stimulus supplied by the 

 ameloblasts. This is in fact the hypothesis formulated by 

 Rose and supported by Skillen. Rose ('91) claims that as soon 

 as growth of the enamel organ ceases rootward, dentin ceases 

 to be formed. Skillen ('21) concludes that the enamel or- 

 gan is a necessary antecedent of dentinogenesis, that it 

 functions in addition to enamel elaboration as a 'stimulative 

 layer' to odontoblast organization and dentin secretion. This 

 hypothesis is sharply contradicted by conditions in the incisor 

 teeth of the white rat. Here the enamel organ orginally caps 

 the dental papilla as in other mammals, but very early, as the 

 tooth elongates, the enamel organ, while still enveloping the 

 base of the persistent papilla, shifts to the front surface of the 

 tooth an4 remains restricted to this side and the coronal apex 

 of the preempted tooth. This accounts for the presence of 

 enamel only on the anterior (outer) surface of the rat's incisor. 

 Nevertheless, dentin is deposited on the inner surface of these 

 teeth in the absence of any representative of the enamel organ. 

 Here again the factor of pressure, that is, adequate resistance or 

 rigidity of tissue, seems to be the essential matter. The meso- 

 blastic tissue of the dental sac is here so dense relatively that 

 it offers the requisite resistance for the deposition and reten- 

 tion of the forming dentin. The supposition of a stimulative 

 factor for dentinogenesis on the part of the radicular portion 

 of the enamel organ seems to have no support from actual histo- 

 genetic facts. 



/. The blood supply of the e7iamel organ 



The central histologic datum of this investigation concerns 

 the question of an intrinsic blood supply of the enamel organ. 

 Where the enamel organ is relatively thin, as on the incisor of 

 the white rat, the matter seems unimportant; for here the cap- 

 illary net, though actually outside of the enamel organ, is never- 

 theless brought so close to the layer of ameloblasts as to be practi- 



