158 



E. Nunn. On the Development of [June 15, 



formed layer of enamel. Huxley* considered it as the persistent 

 basement membrane. 



Fig. 1, A, PI. 2, is a drawing* en face of this membrane as seen upon 

 the molar of a young rabbit • it was taken from the tooth below the 

 gum. The areolae, upon its outer surface, were perfectly clear and 

 transparent, as has been described, and a side view (fig. 1, B) showed 

 them to be elevated into ridges. But the spaces or disks which they 

 bounded were granular and were found to stain slightly with carmine. 

 In this preparation they were round, but in some they are distinctly 

 polygonal, while in others they are faintly seen and their outlines 

 cannot be definitely traced. In the part of this membrane exposed 

 above the gum, which appeared perfectly homogeneous, they could not 

 be detected at all. While this membrane still covers the embryonic tooth 

 " it maybe that a few of the elongated cells of the organon adaniantinaa 

 adhere to it."f This may be the case even when the membrane has been 

 well washed with a camel-hair brush previously to being hardened, as 

 in fig. 1, B. This preparation clearly shows the reticulation to corre- 

 spond in position to the cell-wall of the overlying cells and the enclosed 

 spaces to the protoplasmic contents. The different structure of the 

 clear colourless glassy areolae and the granular slightly coloured disks, 

 proves the reticulation to be something more than the impression of 

 adjoining cells, as it has been described to be. The frequently granular 

 character of the disks is rarely so distinct as in the present preparation, 

 generally requiring a high power of the microscope for its demonstra- 

 tion, and it seems previously to have escaped observation. 



The ridges, always upon the outer surface of the membrane, appear 

 to be the walls of the cells, broken off: a little above the extreme end of 

 the cell, and in many preparations they occupy a larger relative space 

 in the membrane than is shown in fig. 1. 



It would seem as if the cell- wall were thickening about this end of 

 the cell, or the protoplasm were undergoing a differentiation into a 

 substance similar to cell- wall. The ends of the cells of the overlying 

 enamel membrane frequently appear jagged (fig. 2), as also the surface 

 of the membrane directed toward them, while its other surface, or that 

 directed towards the enamel, is perfectly smooth, as is the case with 

 the surface of the enamel, when the tooth has been sufficiently care- 

 fully and gradually decalcified. 



In the upper part of fig. 2, where the tissues have not been separated, 

 no membrane can be detected, the line between enamel cells and mem- 

 brane not being a sharp one ; but when the former are torn away, the 

 latter is isolated (fig. 2, c), and a front view showed it to have the 

 appearance of fig. 1, save that the disks are polygonal (fig. 2, B). 



A similar structure on the tooth of the thornback is shown in fig. 3. 

 * Huxley, I.e. 

 f Huxley, I.e. p. 157. 



