832 EMBRYOLOGY 



two superior projections, which is called the frontal, or intermaxillary 

 process. This extends from the forehead that portion which covers 

 the front of the cerebrum downward. The superior maxillary pro- 

 jections then advance forward, gradually passing to meet the frontal 

 process, but leaving two small openings on either side of the median 

 line, which are the openings of the nostrils. The upper portion of the 

 frontal process thus forms the nose ; but below, is the lower end of this 

 process, which is at first split in the median line, projects below the 

 nose, and forms the incisor process, at the lower border of which are 

 finally developed the incisor teeth. As the superior maxillary processes 

 advance forward, the eyes are moved, as it were, from the sides of the 

 head and present anteriorly, until finally their axes become parallel. 

 These processes advance from the two sides, come to the sides of the 

 incisor process, beneath the nose, unite with the incisor process on 

 either side, and their lower margin, with the lower margin of the incisor 

 process, forms the upper lip ; but before this, the two lateral halves of 

 the incisor process have united in the median line. At the bottom of 

 the cavity of the mouth, a small papilla makes its appearance, which 

 gradually elongates and forms the tongue (see Plate XVI, Fig. 5). 



While this process of development of the anterior portion of the first 

 visceral arch is going on, at its posterior portion the malleus and incus 

 are developing, the former being at first connected with the cartilage of 

 Meckel, which extends along the inner surface of the inferior maxilla, 

 the cartilages from either side meeting at the chin. The cleft between 

 the first and the second visceral arch has closed, except at its posterior 

 portion, where an opening is left for the external auditory meatus, the 

 cavity of the tympanum and the Eustachian tube. 



At the same time the second visceral arch advances and forms the 

 stapes, the styloid ligament and the lesser cornua of the hyoid bone. 

 The third arch advances in the same way ; and the arches from the two 

 sides meet, become united in the median line and form the body and 

 the greater cornua of the hyoid bone. The clefts between the second 

 and the third and between the third and fourth arches finally are 

 obliterated. 



The fourth arch forms the sides of the ne.ck and the larynx, the 

 arytenoid cartilages being developed first. In front of the larynx and 

 just behind the tongue, is a little elevation that is developed into the 

 epiglottis. The 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. 



