THE DEVELOPMENT OF THE TEETH. 155 



Waldeyer and other authorities ; it will be seen that this 

 sheet, produced solely by the destructive action of reagents, 

 corresponds with the membrana preformativa of some writers 

 (see page 171), and with the membrane described by Pro- 

 fessor Huxley as intervening between the enamel cells and 

 the enamel. Hence it will be seen that the fact of acids 

 raising a membrane from the surface of the enamel does not 

 really militate against the theory that the enamel is due to 

 the direct conversion of the enamel organ into enamel. 



The ends of the enamel cells near to the formed enamel 

 are granular, this granularity being due to the deposition of 

 particles of lime salts, as is indicated by its clearing up 

 when treated with acid. 



The cells on the one side of the membrane readily separate 

 from one another, adhering, however, slightly by their 

 dilated ends (vide supra), and the fact that we are able to 

 isolate the youngest layer of enamel as a thin sheet is 

 probably to be explained by its chemical nature. It ap- 

 pears to belong to that class of peculiarly resistant sub- 

 stances which are to be found on the borders of calcification, 

 and behaves very much like Professor Harting's " calco- 

 globulin" (see page 150); at all events it may safely be 

 said to have undergone some chemical change preparatory 

 to the reception of its full amount of lime salts. 



The calcification of the enamel should be so complete 

 that its fibrous structure is but slightly apparent in longi- 

 tudinal sections, and the individual fibres should appear 

 structureless, with the exception of the feebly marked 

 striation (see page 50). In enamel of imperfect structural 

 character the centre of the fibre is not completely calcified, 

 the arrest of development having taken place short of its 

 full conversion. 



The stellate tissue of the enamel organ disappears some 

 time before the whole thickness of the enamel is formed, 

 and changes go on in the latter up to the time of the erup- 



