HISTOLOGY OF THE HARD DENTAL TISSUES. 113 



seated portions of the enamel-germ, the spheroidal cells forming 

 the central part of it begin to increase rapidly, causing the de- 

 pression, which was at first like a tubular gland in shape, to become 

 flask-shaped, the neck being still continuous with the epithelial 

 structure from which it sprang. The dentine germ, arising from 

 the mucous tissue, also begins to enlarge, and shoot upwards till it 

 projects into the base of the enamel-germ, which, growing in a 

 contrary direction, at last invests the dental papilla. 



Histological changes, of a very interesting character, now occur 

 in this enamel- organ ; the cells of it, in immediate contact with 

 the dental pulp, become elongated, and, by mutual pressure, assume 

 the shape of beautiful hexagonal prisms, each being bounded by a 

 decided limiting membrane at the sides, but open at each extremity. 

 Upon the top of the dental papilla the enamel-cells are perpen- 

 dicular, while at the sides they assume a horizontal direction ; in 

 the intermediate positions they may be observed occupying every 

 angle till they join the vertical cells — calcification of the prisms 

 commences at those ends of the prisms which are nearest to the 

 dental pulp, and at the sides of each prism, so that the centres of 

 each are the last portions calcified — calcification proceeds from 

 within outwards and only reaches its maximum of density some 

 months after the tooth is cut. It would be foreign to the nature 

 of this short paper to take up your time by entering into all the 

 abstruse details relative to the changes which now take place in 

 the enamel ; it must suffice if I lay before you this brief outline of 

 its developmental history, leaving to those who feel an interest in 

 the subject the task of reading up for themselves the interesting 

 information to be gathered from the numerous authors who have 

 written especially on this subject. 



While we have been considering the formation of the enamel, 

 other changes have been taking place, to which I must now direct 

 your attention. 



About the period when calcification of the enamel commences 

 the layer of cells covering the external surface of the dental papilla, 

 and to which the name of the Odontoblastic layer may be given, 

 assumes an active condition in forming a cap of dentine imme- 

 diately beneath the developing enamel-fibres, so that if the young 

 tooth, deprived of its soft structures, be examined at this stage it 

 will represent a thin, hollow shell of dentine corresponding in shape 

 to the future tooth. 



