ORGANS OF SPECIAL SENSE 285 



nerve grow into its ventral wall exclusively, between its epithelial 

 cells, which gradually become disarranged and irregular. Thus 

 the ventral wall becomes increasingly thick and the lumen excen- 

 tric. By the sixth day the lumen appears in cross-section as a 

 narrow lenticular space with an epithelial roof, above the large 

 optic nerve. Soon after, the lumen disappears entirely; no trace 

 of its former existence is to be found on the eighth day. 



II. The Development of the Olfactory Organ 



The origin of the olfactory pit, external and internal nares, and 

 olfactory nerve, has already been considered (pp. 169, 215, and 263). 

 Before the formation of the internal and external nares, not only 

 has the entire olfactory epithelium become invaginated, but, owing 

 to the elevation of internal and external nasal processes, the pit 

 has become so deepened that the margin of the olfactory epithe- 

 lium proper now lies a considerable distance within the cavity. 

 That part of the nasal cavity thus lined with indifferent epithelium 

 is known as the olfactory vestibule. After the fusion of the 

 internal nasal process with the external nasal and maxillary 

 processes, the cavity deepens still more. 



The choanse lie at first just within the oral cavity, but the 

 palatine processes of the maxillary process, growing inwards 

 across the primitive oral cavity (pp. 298, 299), unite on the sixth 

 or seventh day at their anterior ends with the internal nasal 

 processes, and thus cut off an upper division of the primitive 

 oral cavity at its anterior end from the remainder; in this way 

 the internal openings of the nasal cavities into the oral cavity 

 are carried back of the primitive choanae; they are henceforward 

 known as the secondary choanse. Further growth of the palatine 

 processes brings them nearly together in the middle line along 

 the remainder of their length, about the eleventh day; but fusion 

 does not take place, the birds possessing a split palate. Thus 

 the superior division of the primitive oral cavity is added to the 

 respiratory part of the nasal passages. 



The nasal cavity is further elaborated between the fourth 

 and eighth days by ingrowths from the lateral wall (turljinals) 

 and by the formation of the supraorbital sinus as an evagination 

 that grows outwards above the orbit. Three turbinals are formed 

 in the nasal cavities, viz., the superior, middle, and inferior tur- 

 binals. These arise as folds of the lateral wall projecting into 



