2IO 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



divided in front, while a 'pseudoconch' (its homology with the 

 superior concha of birds is uncertain) is developed above and behind 

 the true concha. Jacobson's organ occurs only in the squamata, 

 wherejit forms a simple pocket in the primitive position, ventral and 

 medial to the nasal cavity, near the nasal septum (fig. 222). 



Fig. 223. — Olfactory region of hen in longitudinal and transverse section, after 

 Gegenb'aur. c, middle concha; ch, choana; i, inferior (anterior) concha; o, connexion of 

 air cavity of head; p, septum of nose; s, superior concha. 



Three folds are developed on the wall of each nasal cavity in birds, 

 an anterior and inferior concha vestibuli, a middle and a superior 

 fold, the middle supported by the maxillo-turbinal, the superior by 

 the naso-turbinal bones (fig. 223). The vestibular conch lacks ol- 



FiG. 224. Fig. 225. 



Fig. 224. — Model of the nasal cavity of a rabbit embryo, 13)^ mm. head length, 

 after Peter, ch, choana; et, first ethmo-turbinal; j, organ of Jacobson; oj, opening of same; 

 tnt, maxillo-turbinal; nt, naso-turbinal. 



Fig. 225. — Nasal cavity of Erinaceus, after Paulli, showing the foldings of the 

 maxillo-turbinals (w/) and the naso-turbinals {nt); et, ethmo-turbinal. 



factory epitheKum at all times, while it disappears from the middle 

 one after hatching, leaving the upper conch as the sole seat of smell 

 in the adult, which corresponds with the limited olfactory sense in 

 these animals. Jacobson's organ is never developed in the adult, 

 through traces of it appear in the embryos. 



With the great increase of the sense of smell in the mammals the 

 nasal labyrinth undergoes a corresponding complication, and is 



