HISTOLOGY OF ENAMEL ORGAN 381 



weeks of age. The material was fixed in the Zenker-formol 

 mixture of Helly, decalcified in a 2 per cent aqueous solution 

 of nitric acid, imbedded in celloidin, sectioned at 12 /i, and stained 

 with hematoxylin and eosin. For assistance in the preparation 

 of the sections I am indebted to two of my students, Mr, T. D. 

 Jones and Mr. J. W. Hicks. 



The reason for selecting this particular material is the fact 

 that it is representative of two sharply distinct types of teeth 

 classified on the basis of development (extent) of the enamel 

 organ. In certain rodents with persistently growing incisors, 

 of which the white rat serves well as a type, the enamel organ 

 is limited in its extent to the outer surface of these teeth. The 

 enamel organ of these teeth occurs, moreover, as a relatively 

 thin layer, the stellate reticulum being practically lacking. In 

 addition to these contrasting characteristics, it exhibits most 

 conspicuously and in most highly developed form the so-called 

 papillary layer' of Williams. The teeth of certain families of 

 certain other mammalian groups (primates, ungulates, carnivores, 

 and marsupials), of which the cat may serve as a type, are charac- 

 terized in their development by extensive enamel organs com- 

 pletely covering the growing teeth. These enamel organs either 

 lack a papillary layer or have it only very slightly developed. 

 They are, moreover, of relatively great thickness, consisting of 

 an inner enamel epithelium or laj^er of ameloblasts, a thin 

 superjacent stratum intermedium, an extensive stellate reticulum, 

 and an outer tunic of enamel epithelium. The most favorable 

 approach to the problems relating to the enamel organ would 

 seem to be by way of a comparative study of these two sharply 

 contrasting types. Such procedure, providing an enlarged basis 

 of definite and distinctive histologic data, gives greater promise 

 of effecting an explanation and reconcilation of the conflicting 

 opinions concerning the structure and function of the enamel 

 organ. 



THE AMERICAN JOURNAL OF ANATOMY, VOI,. 29, NO. 3 



