OLFACTORY ORGANS. 



537 



thelium. Further changes subsequently take place in connection 

 with the posterior nares, but these are described in the section 

 dealing with the mouth. 



In Mammalia the general formation of the anterior and 

 posterior nares is the same as in Birds ; but, as shewn by Dursy 

 and Kolliker, an outgrowth from the inner side of the canal 

 between the two openings arises at an early period ; and 

 becoming separate from the posterior nares and provided with a 

 special opening into the mouth, forms the organ of Jacobson. 

 The general relations of this organ when fully formed are shewn 

 in fig. 312. 



In Lacertilia the formation of the posterior nares differs in some 

 particulars from that in Birds (Born). A groove is formed leading from 

 the primitive nasal pit to the mouth, bordered on its inner side by the 

 swollen edge of the fronto-nasal process, and on its outer by an outer- 

 nasal process ; while the superior maxillary process does not assist in 

 bounding it. On the inner side of the narrowest part of this groove 

 there is formed a large lateral diverticulum, which is lined by a con- 

 tinuation of the Schneiderian epithelium, and forms the rudiment of 

 Jacobson's organ. The nasal groove continues to grow in length, but 

 soon becomes converted into a canal by the junction of the outer-nasal 

 process with the fronto-nasal process. This canal is open at both ends : 

 at its dorsal end is placed the original opening 

 of the nasal pit, and its ventral opening is 

 situated within the cavity of the mouth. The 

 latter forms the primitive posterior nares. The 

 superior maxillary process soon grows inwards 

 on the under side of the posterior part of the 

 nasal passage, and assists in forming its under 

 wall. This ingrowth of the superior maxillary 

 process is the rudiment of the hard palate. 



On the conversion of the nasal groove into 

 a closed passage, the opening of Jacobson's 

 organ into the groove becomes concealed ; and 

 at a later period Jacobson's organ becomes 

 completely shut off from the nasal cavity, and 

 opens into the mouth at the front end of an 

 elongated groove leading back to the posterior 

 nares. 



In Amphibia the posterior nares are formed 

 in a manner very different from that of the 

 Amniota. At an early stage a shallow groove 



FIG. 312. SECTION THROUGH 

 THE NASAL CAVITY AND JA- 



COBSON'S ORGAN. (From 

 Gegenbaur.) 



sn. septum nasi ; en. nasal 

 cavity ; J. Jacobson's organ ; 

 d. edge of upper jaw. 



is formed leading from the nasal pit to the mouth ; but this groove instead 



