720 MANUAL OF PHYSIOLOGY. 



the ear arises at the side of the face, in connection with one of the 

 visceral clefts. 



The nasal depressions or pits appear in the wall of the head, 

 covering the anterior part of the brain. 



Just above the first visceral arch or fold is the depression which 

 ultimately becomes the buccal cavity, and unites with the alimen- 

 tary tract to form the mouth. 



The first fold is called the mandibular ; this gives off at either 

 end a process which grows upwards and inwards, forming the ru- 

 diment of the superior maxillary bone and side of the face. 



Between these is a median process, the fronto-nasal, which gives 

 off, on the inner sides of the nasal grooves, projections which form 



FIG. 300. 



Different stages of the development of the head and face of a human em- 

 bryo. A. Embryo of four weeks. (Allen Thomson.) B. Embryo of six 

 weeks. (Ecker.) C. Embryo of nine weeks. (Allen Thomson.) a. Au- 

 ditory vesicle. 1. Lower jaw. l x . First pharyngeal cleft. 



the inner nasal processes ; these unite with the superior maxillary 

 processes to close in the nostril and form the lip. 



The outer nasal process is a thickening on the outer side of the 

 nasal depression, which, running down towards the superior max- 

 illary process, forms eventually the lachrymal duct. 



The mandibular arch forms the lower jaw, and between this and 

 the superior maxillary process the buccal cavity is developed 

 chiefly by the outgrowth of the surrounding tissue ; the epiblast 



