1891.] ORGAN IN THE CROCODILIA. 153 



essential functional constituent of Jacobson's organ is to be regarded 

 " as a highly specialized portion of the olfactory epithelium " ^ 

 As it exists among Reptiles it may be defined as a distinct sac lying 

 (on each side) immediately beneath the anterior portion of the nasal 

 chamber, and communicating with the mouth-cavity by means of a 

 delicate duct, which arises posteriorly and passes obliquely down- 

 wards and forwards, immediately external to the vomer, to reach its 

 anterior extremity ^. Examination of any ordinary Lizard will show 

 that the organ itself lies immediately above and to the outer side of 

 the vomer, and that the point of communication between its duct 

 and the mouth lies within the embrace and at the anterior end of 

 a fold of mucous membrane common to it and the posterior nostril. 



Jacobson's organ, as is well known, attains its greatest differ- 

 entiation among Mammals. Its more minute structure has been 

 worked out by Klein ^, Harvey *, and others ; and a more general 

 communication upon it has been recently published by Herzfeld% in 

 which a classification has been attempted, in accordance with the 

 presence or absence of naso-palatine canals and with other leading 

 features described. This organ lies, in Mammals, within the embrace 

 of a cartilaginous sheath {jc, fig. 3) derivative of a downgrowth 

 (ns.") of the alary cartilage of the nose {ns."), and it is in communi- 

 cation with the olfactory chamber by a wide orifice {e.j.) which 

 overlies the naso-palatine (Stenson's) canal (c.s.). The main body 

 of the organ of each side, with its surrounding cartilage, is supported 

 upon a scroll-shaped prolongation of the premasilla {p.m.) usually 

 termed its palatine process (p-p.) This passes back and at the 

 posterior extremity of the entire organ meets a special lobe of 

 the vomer {vo.'") : between the latter bone and the supporting 

 apparatus of the Jacobson's organ there is invariably an intimate 

 relationship. 



Comparison of the Mammal (fig. 3) with the Crocodilian Caiman 

 niger (fig. 2) shows that there is a fundamental similarity in position 

 between the so-called "palatine process" of the premaxilla in the 

 former (p-p.), and that which I have herein termed the palatine lobe 

 of the vomer in the latter (vo.'"). Albrecht®, Sutton'^, and Parker* 

 have directed attention to the fact that the so-called "palatine 

 process "of the Mammalia may be distinct in origin from the body 

 of the premaxilla with which it ankyloses. I have long been 

 suspicious of a similarly distinct origin of the same for the Common 

 Rabbit {Lepus cuniculus) here figured ; and my pupil, Mr. R. H. 

 Burne, who, at my request, has looked into the matter, has shown 

 me that such is the case in embryos measuring 8 centim. iu length. 

 Parker has described the vomer of certain Edentata, Insecfivora, 



1 Eamsay Wright, Zoolog. Anzeiger, 1883, p. 393. 



^ Cf. Leydig, op. cit. pp. 96, 99, and pi. Tiii. 



3 Quart. Journ. Micr. Sci. n. s. vol. xxi. pp. 219 & 549 (1881). 



^ Ibid. vol. xxii. p. 50 (1882). 



^ Zoolog. Jabrb. Ed. iii. p. 551 (1889). 



■"^ Corresp. deutsch. authrop. Gesellscli. Miinchen. 1883, no. 11, p. 170, 



' P. Z. S. 1884, p. .566. 



« Pliil. Tr. pt. i. 1885, pp. 1-275. 



