VOMERO-NASAL ORGAN 95 



of smell located in the respiratory passage and an 

 entirely independent vomero-nasal organ. The latter, 

 in the form of a blind sac, opens into the cavity of the 

 mouth. This peculiarity is probably dependent upon the 

 growth of the hard palate in reptiles whereby a new 

 adjustment between the nasal cavity and the mouth is 

 brought about. 



In mammals the vomero-nasal organ also shows much 

 diversity. It is apparently best developed in the lower 



Fio.121. — Transverse section of the snout of a young frog snowing the pajtial division 

 of the nasal cavity into a lateral or olfactory portion (o) and 

 a median or vomero-nasal portion (v). 



forms, such as the Australian duckbill Ornithorhynchus, 

 and it is rudimentary in such groups as the primates 

 including man. In general it has the form of a blind 

 sac that opens usually by means of the naso-palatine duct 

 (Stenson's duct) into the mouth, a relation that is prob- 

 ably reminiscent of its original connection with the 

 primitive choana of which the naso-palatine duct may 

 be regarded as a trace. Less commonly it opens directly 

 by its own duct into the nasal cavity. This condition 

 obtains in certain rodents such as the rabbit, guinea pig, 

 rat, and mouse, and in certain primates including man. 



