DEVELOPMENT OF THE TEETH. 487 



always that of part of the cell from which the phenomena of life 

 radiate with the greatest activity, whilst the peripheric portions con- 

 stantly, on the other hand, have a tendency to death or to transforma- 

 tion into intercellular substance, etc. In favour of the same view also is 

 the remarkable circumstance, that in all elongated columnar cells, with 

 one nucleus in their interior, the latter is constantly found to occupy 

 the attached and never the free extremity. 



From the foregoing remarks, then, it appears that enamel 

 is to be regarded as the petrified dental epithelium, and that 

 its essential part corresponds to the mucous layer of the oral 

 epithelium, whilst the cuticle, though perhaps by a secondary 

 metamorphosis, is associated with the horny structures. 



The delicate membrane described by Huxley (37), in his account of 

 the structure of the teeth, which can be raised with tolerable facility 

 from the surface of the developing enamel after it has been sub- 

 jected to the action of hydrochloric acid, is the youngest layer of the 

 enamel as yet but slightly impregnated with mineral constituents 

 (Tomes, 29). The foraminated appearance of the membrane is in 

 favour of this view. The enamel cells first undergo petrifaction in 

 their investing (external) zone, the axial portion of the protoplasm 

 retaining its softness for a time, and in isolated cells forming a kind 

 of process (Tomes' Process of the Enamel Cells (49), (see fig. 103, 

 No. 2). As a consequence of this the youngest layer of enamel 

 must necessarily exhibit a number of foramina, corresponding to the 

 " Processes" of Tomes. Huxley correctly identifies this membrane 

 with the membrana praformativa of Kaschkow, but erroneously con- 

 siders the cuticula dentis to proceed from it. Raschkow described a 

 thin homogeneous membrane investing the dentine germ, which was 

 regarded by Todd and Bowman, and by Kolliker, as a basement 

 membrane of the dental papilla covering the surface invested by 

 epithelium (enamel cells). Huxley (37) and Kolliker also describe a 

 basement membrane between the mucous membrane and the external 

 epithelium. Such a membrane is, however, only discoverable when 

 the enamel cells have attained a certain stage of development, and 

 have already begun to be calcified. If this membrane, which ex- 

 hibits the characteristic foramina of Huxley's membrane, be raised 

 by the action of hydrochloric acid, no other homogeneous basement 

 membrane can be demonstrated on the dentine germ. 



The papillary projections of the dental sacculus directed towards 



