162 



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



cells may appear to lie quite upon the cellular pulp ; but if the section 

 be good and sufficiently thin, a close examination will often reveal 

 the membrane upon some part or other of the surface of the pulp. 



Fig. 11 represents a section of a tooth papilla a little more advanced. 

 A thin layer of dentinal basis (neither dentine nor enamel) covers the 

 surface of the pulp, and over all is the basement membrane which, in 

 one part, has fortunately been torn away with needles, and is shown 

 to have a very different appearance from the cuticula, which later is 

 developed above it, and can easily be isolated or "raised up " by acid. 



In the Plagiostomes the soft dentinal basis under the basement 

 membrane increases in thickness to a considerable depth before the 

 beginning of calcific deposit, and thus affords a better opportunity for 

 the study of the behaviour of the basement membrane, as well as for 

 that of the formation of enamel and dentine, than is met with in 

 mammalian teeth. It frequently becomes two or three times as thick 

 as the future enamel will be before the occurrence of any calcification, 

 which at last begins at the surface and proceeds inwards. The base- 

 ment membrane of the derma can readily be traced running up over 

 the surface of the young tooth at all stages, while the tissues are still 

 soft. But its fate, after calcification begins, is more difficult to follow ; 

 whether it calcifies or remains uncalcified, cannot of course be deter- 

 mined from acid preparations, and from these alone it would seem 

 that thin sections could be made. Experiment has proved, however, 

 that alcoholic preparations of considerably calcified young teeth can, 

 by sacrificing a razor for each section, and coming first upon the soft 

 parts, then upon the hard, be cut into fine sections. These sections 

 throw great light upon the manner of calcification and the relation of 

 the soft and hard parts ; they show, too, that after the removal of the 

 cuticula no -zmcalcified membrane remains upon the surface of the 

 tooth, but owing to the great refractive power of the calcified portion, 

 it seemed impossible to demonstrate a thin membrane on its margin. 

 The strong appearance of a membrane might still be simply the re- 

 fractive lines. The persevering and careful use, however, of diluted 

 nitric acid added under the coverslip, brings clearly into view the base- 

 ment membrane upon the outer surface of the tooth ; it is not " raised 

 up," however, as is the case with the cuticula, if it has not been pre- 

 viously removed, and is an altogether different and much more delicate 

 structure than the latter membrane. Fig. 12 is a section prepared in 

 the way just described, showing the basement membrane, and at one 

 place a fragment of the cuticula adhering to it. Teeth far enough 

 developed to show a distinctly differentiated enamel layer with a 

 broad band of dentine underneath, can, with sufficient skill, be cut into 

 the thinnest sections ; and, if these are vertical, the basement mem- 

 brane may always be demonstrated on their surface or under the 

 cuticula if it remain adherent. 



