DEVELOPMENT OF THE FACE. 411 



lungs appear, in the human embryon, during the sixth week. 

 The two portions into which the original bud is bifurcated 

 constitute the true pulmonary structure, and the formation 

 of the trachea and bronchial tubes occurs afterward and is 

 secondary. 1 We have indicated the pulmonary structure as 

 branching processes from the bronchial tubes, merely for con- 

 venience of description. 



Development of the Face. 



The development of the face in the embryon of mammals 

 is somewhat complex, but it is peculiarly interesting, as its 

 study enables us to comprehend the manner in which various 

 very common malformations of the face and palate are pro- 

 duced. The anterior portion of the embryon, as we have 

 seen in studying the development of the trunk, remains open 

 in front long after the medullary plates have met at the 

 back and enclosed the neural canal. The common cavity of 

 the thorax and abdomen is closed by the growth of the vis- 

 ceral plates, which meet in front. These are projecting 

 plates of the intermediate blastodermic layer, which gradu- 

 ally extend forward from the vertebral column. At the same 

 time that the visceral plates are thus closing over the thorax 

 and abdomen, four distinct, tongue-like projections appear, one 

 above the other, by the sides of the neck. These are called 

 the visceral arches, and the slits between them are called the 

 visceral clefts. a The first three arches, enumerating from 

 above downward, correspond, in their origin, to the three 

 primitive cerebral vesicles. The fourth arch, which is not 

 enumerated by some authors, who recognize but three arches, 

 corresponds to the superior cervical vertebrae. Of these four 

 arches, the first is the most important, as its development, in 

 connection with that of the frontal process, forms the face 

 and the malleus and incus of the middle ear ; the second arch 



1 BURDACH, Traite de physiologic, Paris, 1838, tome in., pp. 484, 486. 

 3 These arches correspond to the branchial vascular arches, which will be 

 fully described in connection with the development of the circulatory system. 



