COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



order that water may flow more readily through this pit, a fold is 

 developed on one side of each naris, which practically divides it into 

 two. In many teleosts there is an actual division of each primitive 

 nostril into two, which may be at some distance from each other, 



FIG. 193. Section through the nasal labyrinth of Polypterus. The nerve runs through 



the centre. 



.,: 



" 



J 



FIG. 194. FIG. 195. 



FIG. 194. Head of chick of 5^ days, showing development of oro-nasal canal 

 after Keibel. cf, chorioid fissure; /, thickening for lacrimal duct; n, nasal pit; on, oro- 

 nasal groove. 



FIG. 195.- Model of mouth of Echidna embryo, after Seydel, showing method of in- 

 growth of palatal folds (pf) to cut off secondary nasal passages, ch, primitive choanae; el, 

 egg tooth; /, opening of Jacobson's organ. 



often at the ends of prominent tubes (fig. 192). Inside the nasal 

 capsule the olfactory epithelium is variously folded in order to increase 

 the sensory surface, often forming a labyrinth of considerable complex- 

 ity (fig. 193). 



