416 GENERATION. 



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

 median line. 



It is somewhat difficult to comprehend the exact mode of 

 development of the face by verbal description alone; but it 

 will be readily understood, after the account we have just 

 given, by studying Figs. 46, 47, and 48, copied from the 

 great atlas of Coste. 



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 ; and want of fusion of these pro- 

 cesses, from arrest of development, produces the malforma- 

 tion 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 embryological re- 

 searches, particularly those of Legros and Magitot, have 

 shown that the old idea of the development of the dental 

 papillae in the bottom of a gutter formed at the border of 

 either jaw is erroneous. According to these observers, whose 

 descriptions seem to be exact and reliable, the first appear- 

 ance 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 structure. This band is readily separ- 

 ated by maceration, and the removal of the portion that 

 dips into the maxilla leaves a groove, which is thought by 

 Legros and Magitot to be the explanation of the description 

 of a groove by the earlier writers. 1 This band extends the 

 entire length of the jaws without interruption. Its supe- 

 rior surface is rounded, and that portion which dips into 



1 LEGROS ET MAGITOT, Contributions d T etude du developpement des dents. 

 Journal de Vanatomie, Paris, 1873, tome fx., p. 455. 



