1g2 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, 
Fic. 193.—Section through the nasal labyrinth of Polypterus. The nerve runs through 
the centre. 
Fic. 194. : Fic. 195. 
Fic. 194.—Head of chick of 54 days, showing development of oro-nasal canal 
after Keibel. cf, chorioid fissure; /, thickening for lacrimal duct; 2, nasal pit; on, oro- 
nasal groove. 
Fic. 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 choanz; et, 
egg tooth; 7, opening of Jacobson’s organ. 
an 
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). 
