160 



E.' Nuiin. On the Development of 



[June 15, 



to have the same structure, and it has been possible, in transverse and 

 longitudinal sections, to trace the gradual transition of the enamel 

 cells into a perfectly homogeneous membrane (figs. 6 and 7, Plate 3), 

 the cylindrical cells growing shorter as they approach the crown of the 

 tooth, until, instead of being columnar, they are almost square, and 

 finally flattened, and at last the outlines of the cells quite disappear, 

 and there is left a perfectly homogeneous membrane. 



These changes are not easy to follow ; in many preparations it is 

 impossible to make anything out, and the drawings » have been made 

 from most fortunate preparations selected from some thousands of 

 sections prepared in various ways. 



The cuticula dentis, then, is formed by the metamorphosis of more 

 or less of the enamel cells, and this metamorphosis may begin before 

 any calcification of the underlying dental tissues. In this stage it has 

 been frequently taken for the " newly-formed layer of enamel," for 

 the "basement membrane," and for the "first-formed layer of 

 dentine." 



2. The Basement Membrane and Memhrana Prceformatiua. 



The mucous membrane of the Plagiostomes immediately under 

 the epithelium is frequently more or less laminated, and one of 

 these laniina? bounding the surface of the mucosa has often been 

 described as the " derh " basement membrane. But being only the 

 outer one of a series of lamina?, it represents the basement membrane 

 as simply the margin of the mucosa, with no definite and special 

 structure of its own. 



In order to get at the special structure of the surface of the derma 

 supposed to be bounded by a basement membrane, it is well first to 

 examine portions which have a cellular rather than a distinctly 

 laminated structure, and it is absolutely necessary that the sections 

 should be perfectly vertical to the surface of the derma, the study of 

 which will be much facilitated if the epithelial cells have, for the most 

 part, been previously washed away. 



Fig. 8 is a vertical section of the dermis and of the base of a young 

 tooth of the skate. The section being very thin and perfectly ver- 

 tical, the structure of the cellular granular dermis stands out in strong 

 contrast to the perfectly clear and dense basement membrane, brought 

 into even greater relief by the cleft (<//.), and by its absence from a 

 part of the derma where it has been accidentally torn awav. The 

 basement membrane runs up from the derma over the surface of the 

 young tooth, which, as yet. has no calcific deposit, though there is a 

 thick dentinal basis under the basement membrane, much thicker than 

 the future enamel. It is finely granular throughout, showing no 

 differences of structure in different parts, no signs of tubules, though 



