DEVELOPMENT OF THE SENSE ORGANS 



371 



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. 351 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- 

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

 primitive choana (Fig. 148). Cranially, the epithelial plate is destroyed by in- 

 growing mesoderm of the maxillary process and median nasal process which 



Olfactory 

 epithelium 



Organ of^ 

 Jacobson 



Inferior 

 concha 



Palatine 

 process 



Dental 



Cartilage of 

 nasal Septum 



Jacobson's organ 



Naso-lachn. 

 ' duct 



ongue 



-MecKe/s 

 cartilage 



FIG. 352. Transverse section through the nasal passages and palatine processes of a 20 mm. embryo. 

 In the nasal septum is seen a section of the organ of Jacobson. X 30. 



replaces it and constitutes the primitive palate (Fig. 350 D). The primitive palate 

 forms the lip and 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 choana. 



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. 351 A, B). As the facial region grows and 

 elongates, the primitive choanae become longer and form slit-like openings in the 

 roof of the mouth cavity. By the development and fusion of the palatine pro- 



