Bawden, Nose and Jacobson's Organ. 141 



and trigeminal nerves ; it communicates with the mouth by a 

 special aperture. This definition applies accurately to the ac- 

 cessory nasal chamber of Caecilians, already mentioned ; in no 

 other vertebrates, however, does it retain the character of a kind 

 of separate nasal chamber, but on the contrary, the higher we 

 pass in the vertebrate series, the more does the maxillary cavity 

 become separated physiologically from the olfactory organ ; it 

 loses its olfactory epithelium and finally degenerates into a 

 simple air-sinus. 



Beard says that the so-called Jacobson's organ of Gymno- 

 phiona is not homologous with that of Reptilia but only a sec- 

 ondary olfactory organ, while the two-fold division of the olfac- 

 tory nerve, though comparable, is yet not homologous with the 

 condition found in Reptilia. Again Wiedersheim in his attempt 

 to homologize the outer diverticle which he found in GymnopJii- 

 ona {Ichthyophis) with the Jacobson's organ of Reptilia, found 

 the difficulty confronting him that this diverticle appeared upon 

 the outer side whereas in every case the true Jacobson's organ 

 has been found lying on the inside of the nasal cavity proper. 

 He therefore concludes that there is no homology or if there is 

 he confesses the difficulty, while Beard denys the possibility, 

 calling it a secondary olfactory organ, to prevent confusion. 



Our studies lead us to an entirely different conclusion. We 

 believe that this diverticle which Wiedersheim found, is in the 

 original position of the primitive Jacobson's organ as an outer 

 diverticle, and that in the course of its evolution it revolves from 

 without, inward (i. e. from laterad, entad) until it holds the po- 

 sition seen in other Amphibia and Reptilia: viz. on the median 

 aspect of the olfactory cavity proper. This, of course, would 

 place Gymnophiona in this respect lower than Anura and possi- 

 bly lower than Urodela in the phylogenetic series. There is 

 some evidence, indeed, for believing that Urodela are but an 

 arrested stage of a higher type — a higher amphibian arrested 

 permanently in its larval condition. At any rate the adult con- 

 ditions as found in Urodela are undoubtedly a transition stage 

 from Gymnophiona and Caecilians and other lower types of 

 Amphibia to Anura and the higher types. 



