828 GENERATION. 



openings of the nostrils appear during the second half of the second month. 

 A little elevation, the nose, Appears between these openings, and the nasal 

 cavity begins to be separated from the mouth. The lips are distinct during 

 the third month, and the tongue first appears in the course of the seventh 

 week. 



When, by an arrest of development, the superior maxilla on one side fails 

 to unite with the side of the incisor process, there is the very common de- 

 formity known as single harelip. If this union fail on both sides, there is 

 double harelip, when the incisor process usually is more or less projecting. 

 As a very rare deformity, it is sometimes observed that the two sides of the 

 incisor process have failed to unite with each other, leaving a fissure in the 

 median line. 



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 sepa- 

 ration between the mouth and the nares ; and want of fusion of these pro- 

 cesses, from arrest of development, produces the malformation known as 

 cleft palate, in which the fissure is always in the median line. 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 rounded 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 removal of the portion 

 that dips into the maxilla leaves a groove. This band 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. 



As 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 jaws. It is thin, flattened, with its free edge curved 

 inward and toward the jaw, and is composed at first of a central layer of 

 polygonal cells, covered by 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 margin 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 as follows : 



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 en- 

 largement enclosed completely in a follicle. This is the dental follicle, and 

 it has no connection with the wedge-shaped band described first. While 



