924 



GENERATION. 



\ 



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. 



The above sketch of the mode of develop- 

 ment of the face enables us to understand the 



/Si origin of certain of the more common malfor- 



mations of this part. When, by an arrest of 

 development, the superior maxilla on one side 

 fails to unite with the side of the incisor 

 process, we have the very common deformity 

 known as single hare-lip. If this union fail on 

 both sides, we have double hare-lip, when the 

 incisor process is usually more or less project- 

 ing. As a very rare deformity, it is sometimes 

 observed that the two sides of the incisor pro- 

 cess have failed to unite with each other, leav- 

 ing a fissure in the median line. 



It is somewhat difficult to comprehend the 

 exact mode of development of the face by ver- 

 bal description alone; but it will be readily 

 understood, after the account we have just 

 given, by studying Figs. 303, 304, and 305, 

 copied from the great atlas of Coste, and plates 

 I. and II., Figs. A, B, C, and D, facing page 

 920. 



The palatine arch is developed by two pro- 

 cesses, which arise on either side from the in- 

 cisor 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 ; and want 

 of fusion of these processes, from arrest of de- 

 velopment, 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. Recent enibryological researches have shown that the old 

 idea of the development of the dental papilla in the bottom of a gutter formed at the 

 border of either jaw is erroneous. According to the most recent observers, 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 each jaw, 

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

 ure. This band is readily separated by maceration, and the removal of the portion that 

 dips into the maxilla leaves a groove, which is thought to be the explanation of the 

 description of a groove by the earlier writers. 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. 



FIG. 80S. Mouth of a human embryon of from 

 twenty-five to twenty-eight days ; magnified \f> 

 diameters. (Coste.) 



1, median or frontal process, the inferior portion of 

 which is considerably enlarged ; 2, right nostril ; 

 3. left nostril; 4, 4, inferior maxillary processes, 

 already united in the median line ; 5, 5, superior 

 maxillary processes, which have become quite 

 prominent and have descended to the level of the 

 slope of the frontal process ; 6, mouth ; 7, first vis- 

 ceral arch; 8, second visceral arch ; 9, third visceral 

 arch; 10, eye; 11, ear. 



