HISTOLOGY OF ENAMEL ORGAN 389 



b. The papillary layer 



Williams was the first to direct special attention to the so-called 

 papillary layer of the enamel organ. It is best developed in 

 rodents. In the tooth of an embryo lamb Williams describes 

 a marked tendency on the part of the cells of the stratum inter- 

 medium to assume 'a more or less orderly arrangement about 

 the capillary loops.' He thinks it 'perfectly evident that in the 

 development of enamel the cells of the stratum intermedium play 

 the most important part in the selection from the blood of the 

 materials for the construction of this tissue.' Williams accord- 

 ingly derives the papillae of the papillary layer from the cells of 

 the stratum intermedium. Skillen, on the contrary, derives 

 them from an 'apposition of outer and inner tunics, which takes 

 place gradually.' According to Skillen, calcification of the en- 

 amel prisms begins only at the time of the formation of the papillary 

 layer. But Williams states that the formation of enamel begins 

 before the full development of the stratum intermedium (there- 

 fore, before the papillary layer is formed) , and that 'it is highly 

 probable, as has been pointed out by previous writers, that the 

 material for the commencement of enamel formation is stored 

 in the stellate reticulum' (p. 118). 



Williams describes the prisms of definitive enamel as the prod- 

 uct of two distinct elements: 1) globules formed in the amelo- 

 blasts, probably directly from the nuclei; 2) 'an albumen-hke 

 appearing substance which is seen passing out of the ameloblastic 

 cells of the stratum intermedium and evidently contains the min- 

 eral matter of which completed enamel consists' (p. 477). Accord- 

 ing to Williams, therefore, the stratum intermedium is of primary 

 importance in amelogenesis. In it he claims are early developed 

 numerous capillaries. In rodents the vascularized layer forms 

 'a highly differentiated secreting tissue.' The cells of the stratum 

 intermedium become arranged in the form of papillae around and 

 between which end abundant capillary loops. Each papilla 

 is said to supply about twenty ameloblasts. 



From the description of the comparative histology of the en- 

 amel organ of the incisors of the rat and of the cat it is clear that 



