DEVELOPMENT OF THE TEETH. 1561 



appearing at the margin of the enamel-organ, where the outer and inner layers of the 

 latter are continuous. The modified epithelial tissue of the middle layer, sometimes 

 called the eyiamel-pidp, is greatest in amount just prior to, or during the beginning 

 of, active tooth-formation, about the fifth or sixth foetal month. 



The inner layer of the enamel-organ comprises a single row of closely set, tall, 

 cylindrical elements, the enamel-cells, adamantoblasts, or ameloblasts, through the 

 active agency of which the enamel is produced. The ameloblasts are best developed 

 where they cover the apex of the dental papilla, the location of the earliest formed den- 

 tine ; in this situation the cells measure from .025-040 mm. in length and from 

 .004-007 mm. in breadth. They possess an oval nucleus about .010 mm. long, 

 which usually lies close to the outer end of the cell, embedded in cytoplasm exhibit- 

 ing a reticulum and often minute granules. The ameloblasts are united with one 

 another by a small amount of cement-substance, and are defined from the interme- 

 diate layer by a fairly distinct border. Opposite the sides of the dental papilla, cor- 

 responding to the limits of the future crown, the ameloblasts gradually diminish in 

 height until they are replaced by low cubical cells which, at the margin of the enamel- 

 organ, are continuous with the epithelium of the outer layer. Preparatory to the for- 

 mation of the dentine of the tooth-root, this margin grows downward towards the 

 base of the elongating dental papilla, which is thus embraced by the extension of the 



Fig. 1320. 



Dental groove 

 Oral epithelium 



Atrophic epithelial net-work __5,igL^ 



Epithelial sheath .- .W^ Position of mesoblastic dental papilla 



3 



Reconstruction of developing lower incisor tooth from embryo of 30 cm. length, about twenty-four weeks. 



[Drawn frotn Rose's model.) 



enamel-organ. The investment thus formed constitutes the epithelial sheath (Fig. 

 1320), a structure of importance in determining the form of the tooth, since it serves 

 as a mould in which the young dentine is subsequently deposited ; there is, however, 

 insufficient evidence to regard the epithelial sheath as an active or necessary factor in 

 the production of the dentine. 



The formation of the enamel, in contrast to that of the dentine, results from 

 the activity of ectoblastic epithelium, and may be regarded as a cuticular development 

 carried on by the ameloblasts. The earliest stage in the production of enamel is 

 the appearance of a delicate cuticular zone at the inner end of the ameloblast ; this 

 fuses with similar structures tipping the adjoining cells to form a continuous homo- 

 geneous mass. The latter soon exhibits differentiation into rod-like segments, the 

 enamel-processes, or processes of Tomes, which are extensions from the ameloblasts 

 and are the anlages of the enamel-prisms, and the interprismatic substance. The 

 latter becomes greatly reduced in amount as the development of the enamel-columns 

 progresses ; the major part, becoming incorporated with the processes of Tomes, 

 forms the cortical portion of the enamel-prisms, while the remainder persists as the 

 cement-substance which exists in meagre quantity between the mature prisms. The 

 enamel-processes are for a time uncalcified, but with the more advanced formation of the 

 enamel-prisms the calcareous material, which is deposited as granules and spherules, 

 appears first in the axis of the prism, later invading the periphery (Ebner). The 



