DEVELOPMENT OF THE FACE 



MS 



nasal processes by the development of the olfactory pits. The processes are dis- 

 tinct and most prominent at 12 mm. (Fig. 149 A). Soon, in 13 to 14 mm. embryos, 

 the median nasal processes fuse with the maxillary processes of the first arch and 

 constitute the upper jaw (Fig. 149 B). The lateral nasal processes fuse with the 

 maxillary processes and form the cheeks, the lateral part of the Hps, and the ate 

 of the nose. Later, the median nasal processes unite and become the median 

 part of the upper lip. Meanwhile the mesial remainder of the original frontal 

 process (Fig. 149 .4) is compressed and becomes the septum and dorsum of the 

 nose. The development of the olfactory organ will be traced on p. 371. 



Lateral nasal process 



Olfactory pit 



Medial nasal process 



Mandible 



Branchial arch II 

 Ventral aorta- 



Eye 



Lacrimal groom 



Maxillary process 



Branchial cleft I 

 . Branchial cleft JI 



Lateral nasal process 



Maxillary process 



Mandible 



Branchial cleft 1 



External naris 



Eye 



Medial nasal process 



Oral cavity 



External ear 



Fig. 149. — Two stages showing the development of the face in pig embryos. X 7. A, Ventral view of 

 face of a 12 mm. embryo; B, of a 14 mm. embryo. 



The early development of the face is practically the same in human embryos 

 (Figs. 150 and 370). In embryos of 8 mm. the lateral and median nasal processes 

 have formed. The maxillary processes next fuse with the nasal processes, after 

 which the median nasal processes unite. Coincident with these changes the 

 mandibular processes fuse and from them a median projection is developed which 

 forms the anlage of the chin. ' 



Epithelial ingrowths begin to form the hps at the fifth week (Fig. 159). As the median 

 nasal processes and the maxillary processes take part in their development, the failure of these 



