140 Journal of Comparative Neurology 



end at their base. In this he finds a close resemblance as in 

 other respects to the olfactory epithelium proper. This crypt- 

 like arrangement of the ganglionic columns of the olfactory 

 epithelium as found in the Ophidia Beard also points out in 

 an obscured situation in Lacertilia; and we have found it present 

 also in several species of Amphibia. His conclusion in general 

 is that Jacobson's organ in Lacertilia and Ophidia is a very highly 

 specialized portion of the epithelium, and says that it, in these 

 cases, at least, must be an incomparably finer sense-organ than 

 their nose proper (No. 1.) On the structure of Jacobson's 

 organ we have investigations by Klein, Leydig, Wright, Born 

 and Solger, but as to the important question of its real nature 

 and origin these are sadly deficient. 



V. Evolution and Development of Jacobson's Organ in Par- 

 ticular. 



Jacobson's Organ is without doubt as Beard says, "a 

 specially differentiated part of the nose." It was discovered 

 by Jacobson, the Danish anatomist, at the beginning of the 

 present century. We will here endeavor to make a comparison 

 of the different types possessing what we think to be the funda- 

 ment or proton (Anlage) of a Jacobson's organ. In general it 

 is safe to assert thst no Jacobson's organ appears in any type 

 lower than fishes, if indeed in fishes there can be found any true 

 homologue of Jacobson's organ. In Polyptetus, indeed (the 

 most highly developed of fishes) there is a short distinct oval 

 sac in the median line which has no connection with the rest of 

 the olfactory, and which receives a special branch of the olfac- 

 tory nerve, but it is doubtful if this is to be homologized in any 

 sense with the true Jacobson's organ as found in the higher 

 types. 



In Caecilians there is a sac having a separate opening in the 

 mouth, but this is to be homologized with the maxillary sinus 

 (air-chamber) of higher vertebrates. Jacobson's organ is de- 

 scribed by one author as a paired accessory nasal cavity, which 

 in an early embryonic stage becomes entirely separated off from 

 from the nasal chamber, and which is supplied by the olfactory 



