DEVELOPMENT OF THE TEETH 833 



The palatine arch is developed by two processes, which arise on 

 either side, from the incisor process, pass backward and upward and 

 finally meet and unite in the median line. The union of these forms the 

 plane of separation between the mouth and the nares. At the same 

 time a vertical process forms in the median line, between the palatine 

 arch and the roof of the nasal cavity, which separates the two nares. 



Development of the Teeth. The first appearance of the organs for 

 the development of the teeth is marked by the formation of a cellular 

 projection extending the entire length of the border of either jaw, which 

 forms a rounded band above and dips down somewhat into the subjacent 

 structure. This band is readily separated by maceration, and the re- 

 moval of the portion that dips into the maxilla leaves a groove. It 

 extends the entire length of the jaws without interruption. Its superior 

 surface is rounded, and that portion which dips into the subjacent 

 mucous structure is wedge-shaped, so that its section has the form of a V. 



So soon as this primitive band is formed which occurs at the sixth 

 or seventh week a flat band projects from its internal surface, near the 

 mucous structure, which is called the epithelial band. This also extends 

 over the entire length of the jaw. It is thin, flattened, with its free 

 dge curved inward and toward the jaw, and is composed at first of a 

 central layer of polygonal cells covered with a layer of columnar 

 epithelium. 



At certain points these points corresponding to the situation of the 

 true dental bulbs there appear rounded enlargements at the free mar- 

 gin of the epithelial band just described. Each one of these is developed 

 into one of the structures of the perfect tooth. The mechanism of the 

 formation of this, which is called the enamel-organ, and of the dental 

 bulb is the following : 



A rounded enlargement appears at the margin of the epithelial band. 

 This soon becomes directed downward adapting the description to the 

 lower jaw and dips into the mucous structure, being at first connected 

 with the epithelial band by a narrow pedicle which soon disappears, 

 leaving the enlargements enclosed completely in a follicle. This is the 

 dental follicle, and it has no connection with the wedge-shaped band 

 described first. While this process is going on, a conical bulb appears 

 at the bottom of the follicle. The enamel-organ, formed from the 

 epithelial band, becomes excavated, or cup-shaped, at its under surface, 

 and fits over the dental bulb, becoming united to it. 



The tooth at this time consists of the dental bulb, with the enamel- 

 organ closely fitted to its projecting surface. The enamel-organ is 

 developed into the enamel; the dental bulb, which is provided with 

 vessels and nerves, becomes the tooth-pulp ; and on the surface 

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