THE TEETH 



173 



The true dental lamina is concerned witli the enamel. Along its attach- 

 ment to the oral epithehum a furrow is formed, called the dental groove, and 

 its deep margin is received into the mesoderm of the embryonic jaw. 



The true dental laminae, which are of ectodermic origin, are at first un- 

 broken, but they soon become broken up into separate masses, called enamel 

 organs, there being twenty of these for the temporary teeth, ten upper and 

 ten lower. Each enamel organ appears as a thickening or protuberance of 

 ectodermic cells on the deep margin of the true dental lamina, which pro- 

 tuberance soon becomes flask-shaped, the neck being directed superficially. 

 1 his neck soon disappears, and the connection between the enamel organ and 

 the ectodermic cells of the free surface of the gum is thereby severed. More- 

 o%'er, the enamel organs become isolated from one another. 



The dental papilla appears as an elevation of the mesoderm of the floor 

 of the dental groove, subjacent to each enamel organ. The number of 

 papillae, therefore, corresponds to the number of enamel organs. As the dental 

 papilla grows, it becomes capped by the enamel organ, which also invests it 

 laterally. The enamel organ thus becomes moulded over the dental papilla 

 in the form of a cap. 



Dental Ridge 



Superficial Layer of Epitheliuoi 

 Deep Layer of Epithelium 

 Dental Ridge [ 



Enamel Germ • ; 



Superficial Layer of Epithelium 

 Deep Layer 



Neck of Enamel Germ 



Tooth-sac (Inner Layer) 



Outer Layer 

 of Growing Tooth-sac 



— Dental Germ 



Enamel Organ : 

 Dental Germ 



Tooth-sac (Outer Layer) 



Bone 



Enamel Org 

 Tooth-papilla 



Fig. 



97. — Development of Tooth (Frey, after Thiersch) 

 Vertical Transverse Section of Upper Jaw. 



Enamel Organ. — As stated, the enamel organ is of ectodermic origin being 

 denved from the true dental lamina, which, in turn, is derived from the 

 ectodermic cells covering the surface of the gum. The chief cells of the 

 enamel organ are situated deeply, and lie close to the surface of the dental 

 papilla. They are long columnar cells, which are called enamel cells, ada- 

 mantoblasts, or ameloblasts, and they give rise to the enamel prisms 



Formation of Enamel Prisms.— Each enamel cell gives rise to one enamel 

 prism, ihe deep part of each cell undergoes calcification. The superficial 

 ends of the enamel cells do not become calcitied, but form a delicate mem- 

 brane, called the enamel cuticle, or Nasmyth's membrane, which covers the 

 crown of the tooth for a short time after its eruption. 



Dental Papilla.— As stated, the dental papilla is of mesodermic origin and 

 It gives rise to the dentine and dental pulp. Essentially, the papilla is a 



