218 TOMES, ON THE DEVELOPMENT OF THE ENAMEL. 



tema, which connects into a mass the cells of the pulp. I do 

 not, however, propose to go into the development of dentine 

 in the present article ; hence the question of the presence or 

 absence of a preformative membrane extending over the den- 

 tinal pulp, and the relations of such membrane to the develop- 

 ment of dentine, may be left for future discussion. 



If attention be directed to the cusps in which calcification 

 has commenced, appearances similar to those described in the 

 incisor will, if similarly treated, be found, excepting only 

 the enamel organ, the columns of which, in this case, are 

 shorter than those in the more advanced tooth. 



Although I have confined the description to the structural 

 conditions found in developing teeth in one jaw, my exami- 

 nations have been extended over the teeth from many fcetal 

 jaws. The results have, however, been uniformly similar. 



Assuming that the foregoing observations have been cor- 

 rectly made, we need have no difficulty in explaining the 

 manner in which the enamel is developed, and in accounting 

 for the appearances exhibited in the fully-formed tissue ; of 

 which a description and figures were given in the last Number 

 of this Journal. The columns of the enamel organ must be 

 regarded as subservient to the development of the fibres, the 

 conversion of the one into the other taking place in the fol- 

 lowing manner : — Tlie proximal end of the column becomes 

 calcified, not uniformly throughout its thickness, but the 

 outer surface or sheath first receives the salts of lime, and at 

 the same time the columns become united laterally. At 

 this point — that is, at the extreme margin of calcification — 

 the columns readily separate from the fibres, and leave a sur- 

 face which, when looked upon directly, has the appearance of 

 a membrane, the reticulate character of which (figs. 4 and 5) 

 is due to the withdrawal of the central portion of the calci- 

 fying column, this central portion being the process which 

 has been described as forming part of the detached column 

 (fig. 2). The calcification of the central part of the column 

 goes on gradually, but does not keep pace with that of the 

 sheath, and when calcified, presents some points of difference 

 when compared with the surface of fibre. Thus, in adult 

 tissue, the interior of the fibre dissolved before the surface, 

 leaving the reticulated appearance described and figured in 

 the last Number of the Journal. Before calcification, the 

 nuclei of the column appear to break up into subgranular 

 matter, which may often be detected at the distal ends of the 

 forming-enamel fibres (fig. 3). The situation usually occupied 

 l)y well-marked oval nuclei is the distal extremities of the 

 enamel-organ columns ; but sometimes we find examples in 



