DEVELOPMENT OF THE SENSE ORGANS 



373 



When the ventro-lateral ends of the median frontal process enlarge 

 and become the median nasal processes they fuse with the lateral nasal 

 processes and reduce the size of the external nares (Pig: 370 B). Exter- 

 nally, 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, membranous structures caudally, and, rup* ur- 

 ing, produce two internal nasal openings, the primitive choance (Fig. 153). 

 Cranially, the epithelial plate is split by ingrowing mesoderm of the maxil- 

 lary process and median nasal process which replaces it, thereby forming 

 the primitive palate (Fig. 369 D). The primitive palate forms the lip and 



Nasal septum 



Ext. naris 



Oral cavity 



Maxillary 

 process 



Mandibl 



Med. nasal process 



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 1 1.3 embryo (after Rabl). 



the premaxillary palate. The nasal fossae now open externally through 

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

 the primitive choanas. 



Coincident with these changes, the median frontal process has be- 

 come relatively smaller, and that portion of it between the external nares 

 and the nasal fossas forms the nasal septum (Fig. 370). As the facial re- 

 gion grows and elongates, the primitive choanae becomes longer and form 

 slit-like openings in the roof of the mouth cavity. By the development 

 and fusion of the palatine processes (described on p. 148) the dorsal por- 

 tion of the mouth cavity is separated off and constitutes the nasal passages 

 (cf. Figs. 371 and 372). The nasal passages of the two sides for a time 

 communicate through the space between the hard palate and the nasal 



