THE SENSE-ORGANS 483 



in the embryo of turbinalia that never develop, and is indicated 

 also by the simple condition of the cavities, and the lack of 

 complexity in the turbinalia. 



An interesting bit of morphology in connection with the his- 

 tory of the nose is that of Jacobson's organ (vomero-nasal 

 organ), at first a sinus or pocket leading out of the main nasal 

 cavity, later an independent organ, and finally a rudiment. 

 This organ is first seen in urodeles, where it appears upon the 

 medial side of the nasal cavity and gradually migrates along 

 the floor, attaining ultimately a lateral position, though still 

 included w r ithin the nasal capsule. This migration is seen by 

 comparing the lower with the higher members of the order 

 and actually takes place during the embryological development 

 of such a form as Triton, where it may be followed step by 

 step. When its lateral position is fully established, it gradually 

 restricts its communication with the main cavity until it is 

 connected by a small duct in the region of the posterior nares, 

 as in Gymnophiona. In Hzards and snakes, where it reaches 

 its highest degree of development, it forms upon each side a 

 tubular, somewhat contorted organ, with a blind anterior end, 

 opening posteriorly into the roof of the mouth by an inde- 

 pendent opening, yet still near the posterior nares. Its ventral 

 wall is rolled up into its thickened and strongly convex 

 dorsal one, and this latter possesses olfactory sense-cells. The 

 position of the organ has again changed owing doubtless to 

 the development of related parts, and it lies almost directly 

 beneath the primary nasal cavity, between it and the hard 

 palate, and thus more nearly in its original position near the 

 median line. 



In turtles, crocodiles, and birds, Jacobson's organ exists 1 

 only in the form of embryonic vestiges, but, on the other hand, 

 K i& luLdlxd upun cither skle^oMhc, carlrtegiTiOtts-iiasal septitm^ 

 a definite organ and persisting throughout life in many cases. 



is located upon either side of the cartilaginous nasal septum, 

 and is protected by a cartilage of its own, the paraseptal, vo- 

 mero-nasal, or Jacobson's, cartilage. When well developed the 

 organ is in the form of a short tube, which opens anteriorly 



