THE SKIN AND SENSE ORGANS 303 



it as analogous to but not homologous with a true Jacobson's organ. 

 Three years later, in 1901, Hinsberg studied the development of the 

 nasal capsule in Triton and came to the conclusion that, in that 

 animal, a patch of sensory epithelium, homologous with Jacobson's 

 organ, arises mesially and shifts to a position lateral to the internal 

 nares hejore this opening develops. Thus he maintains that the 

 lateral position is a specialization of the Urodela, but that neverthe- 

 less the homology is true. Zuckerkandl (19 10) agrees in the main 

 but concludes, on the grounds of its nerve supply, that the Jacobson's 

 organ of Urodeles cannot be homologous with that of the Amniota. 

 Bruner (19 14) agrees with Seydel as to the location of Jacobson's 

 organ, while Herrick (19 14), also on neurological grounds, doubts 

 the existence of this structure in the Urodela, but says that in Anura 

 it has assumed a definitive form as a diverticulum from the mesial side 

 of the nasal sac, with its characteristic innervation by the vomero-nasal 

 nerve. Von Navratil (1926) does not agree with Seydel's identifica- 

 tion of the sinus lateralis nasi with the organ of Jacobson on morpho- 

 logical grounds, but thinks that Seydel's sensory epithelium may 

 become the sinus maxillaris. In 1927 Kurepina investigated the 

 development of the primary choanae in Urodeles (Triton) and 

 arrived at conclusions diametrically opposed to those of Hinsberg. 

 He claims the existence of a primary oro-nasal groove between the 

 nose and mouth, and hence there can be no lateral shifting of Jacob- 

 son's organ such as Hinsberg describes. 



Thus, while it still remains a somewhat doubtful question, there 

 is evidently a growing consensus of opinion among morphological 

 embryologists against Seydel's view, and in favour of the absence of 

 Jacobson's organ in Urodela. The absence of a vomero-nasal nerve 

 may perhaps be taken as the deciding factor. It is generally agreed 

 however that the sinus lateralis nasi is t}\e functional equivalent o^ the 

 Amniote organ of Jacobson, i.e. that it is used for testing the ex- 

 halent current. 



3. Opening and Closing Mechanism. 



Another region of special interest in the nasal organ is the 

 apparatus for opening and closing the external nares. This was dis- 

 covered and described by Bruner (1896 and 1901) and consists of 

 a set of three smooth muscles, one constrictor and two dilatators. 

 There is no cartilaginous support to the edge of the narial opening 

 as there is in the Frog. The opening in the cartilaginous nasal cap- 

 sule, in which each external naris lies, is very much larger than the 

 actual cutaneous opening, and houses the muscular apparatus as well. 



