358 THE FORMATION AND STRUCTURE 



Structureless Basement Membrane, or Membrana 



Pr^.maformativa. 

 By means of selective stains 1 have been able to demonstrate 

 the existence of a membrane-like structure lying between the 

 ameloblasts and the forming enamel, and also between the amelo- 

 blasts and the stratum intermedium. It is uncertain whether this 

 structure is identical with the membrana prsemaformativa of 

 Raschow, Huxley, and others. Its existence has been disputed 

 by several of the more recent writers upon this subject ; but it is 

 plainly shown in Figs. 3, 4, 5, 6, and 7. I suggest that the struc- 

 ture lying between the ameloblasts and the stratum intermedium 

 be called the outer ameloblastic nmnbrane, while that which sepa- 

 rates the enamel cells from the forming enamel may be known as 

 the inner ameloblastic membrane. 



The Cells of the Stratum Intermedium. 

 It has for many years seemed clear to me that the enamel 

 organ, and more especially the cells of the stratum intermedium, 

 should be classed among the true secreting tissues. I am not able 

 to understand how so many good observers have failed to see the 

 intrinsic plexus of blood vessels, which is very early developed in 

 this layer of cells. Wedl, Magitot and Legros, Sudduth, and 

 others say that they have uniformly failed to detect a blood 

 supply within any part of the enamel organ proper. If we exa- 

 mine the evolution of this tissue in the enamel organ of the 

 mouse or the rat, its place among glandular structures at once 

 becomes evident. All appearance of round, polygonal, or oblong 

 cells, as usually seen in the stratum intermedium, has entirely 

 disappeared, and in their place we have a highly differentiated 

 secreting tissue. The ameloblasts are surmounted by epithelial 

 papillge, around and between which is a free distribution of capil- 

 lary loops. The ameloblasts are seen to be in intimate relation 

 with the papillae, each ameloblast seeming to have a root-like 

 process which extends into and is lost in the substance of the 

 papilla to which it belongs. The diameter of each papilla is equal 

 to about that of five or six ameloblasts, and each papilla may 

 therefore be said to supply from twenty to twenty-five ameloblasts. 

 Blood-vessels are seen, in both transverse and longitudinal section, 

 everywhere about the papillae, and we may often observe branches 



