DEVELOPMENT OF THE SENSE ORGANS 



373 



nasal grooves into blind pits or fossae, shutting them off from the mouth cavity 

 (Fig. 370). Thus in embryos of 10 to 12 mm. the nasal fossa has but one opening, 

 the external naris, and is separated from the mouth cavity by an ectodermal plate 

 (Fig. 369 D, E). 



The ventro-lateral ends of the median frontal process enlarge and become 

 the median nasal processes which fuse with the lateral nasal processes and re- 

 duce the size of the external nares (Fig. 370 B). Externally, the nares are now 

 bounded ventrally by the fused nasal processes. The epithelial plates which 

 separate the nasal fossae from the primitive mouth cavity become thin membra- 



Nasal septum 



Ext. naris 

 Lat. nasal 

 ■process 

 Med. nasal 

 process 

 Maxillary- 

 process 



Mandible 



Ext. naris- 



Lat. nasal, 

 process 



Oral cavity 



Maxillary 

 process 



Mandible' 



Med. nasal process 



B 



Oral cavity 



Fig. 370. — Two stages in the development of the jaws and nose. A, Ventral view of the end of the head 

 of a 10.5 mm. human embryo (after Peter); B, of an 11.3 mm. embryo (after Rabl). 



nous structures caudally, and, rupturing, produce two internal nasal openings, the 

 primitive choana (Fig. 153). Cranially, the epithelial plate is destroyed by in- 

 growing mesoderm of the maxillary process and median nasal process which 

 replaces it, thereby forming the primitive palate (Fig. 369 D). The primitive 

 palate forms the lip and the premaxillary palate. The nasal fossae now open ex- 

 ternally through the external nares and internally into the roof of the mouth 

 cavity through the primitive choance. 



Coincident with these changes the median frontal process has become rela- 

 tively smaller and that portion of it between the external nares and the nasal 

 fossae becomes the nasal septum (Fig. 370). As the facial region grows and elon- 



