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Show 156 PROF. G. B. HOWES O N JACOBSON'S [Feb. 17, under elongation of the snout, with its accompanying specialization) in the manner suggested by Parker himself for the " inter-palatines " of Tarsipes1. IV. The foregoing considerations justify us in regarding the palatine process of the mammalian premaxilla, which, be it remembered, ensheaths the organ of Jacobson, and the palatine lobe of the vomer of Caiman niger as, at least provisionally, one and the same element; and it is necessary now to turn to the latter, by way of inquiring how far its inner capsule may or may not be found to agree with that of the organ named. There can be now no doubt that that structure so :netimes referred to in the Amphibia as an organ of Jacobson is a maxillary sinus, non-homologous with the Jacobson's organ of the higher Ver-tebrata. The latter exists, in that which the known facts of development show to be most nearly its original form, among the Lacertilia. Its general relationships in these animals have been already described (above, p. 153). That the naso-palatine canal of the quadrupedal mammals (c.s., fig. 3) is the representative of the closed duct of the Lizards can hardly be doubted, on comparison of the two types ; it has been shown by Herzfeld to be regularly absent in some mammals, while the aperture of communication between the body of Jacobson's organ and the floor of the nasal chamber (a.jj, fig. 3), met with in most mammals, is unrepresented in reptiles. On a survey of the known facts, I incline to the belief that the development of the last-named orifice is to be correlated with the loss of communication between the naso-palatine duct and the body of the organ, and that its appearance may have led up to that suppression of the duct in question seen in some forms (ex. Equus~). Be this supposition worth what it may, the accepted principles of morphology forbid our looking upon the Jacobson's organs of Reptiles and Mammals as in any way distinct. The vomerine bulla of Caiman niger (vo.'", figs. 2 and 5) occupies an essentially similar position to the body of Jacobson's organ in both Mammals and Reptiles. Its aperture of communication with the nasal pharynx lies, like that of the duct of the Jacobson's organ in the Lizards (and less conspicuously, but no less surely, like that of mammals), within the area of the true posterior nostril. As I have been unable to examine a spirit-preserved head I am not in position to state whether, as seems not unlikely, a tubular duct may have arisen at this point or not. While, however, I have been unable to detect in any other Crocodile the entrance thereabouts of any nerve or blood-vessel such as might conceivably have been transmitted by it, sufficient of the dried remains of its lining membrane was present to enable me to assert, with assurance, that it contained a prolongation of the olfactory mucous membrane. This being so, its orifice might be not inaptly compared either to that of the Jacobson's organ of a 1 Stud. Mus. Univ. Coll. Dundee, vol. i. p. 80 (1890). 2 Herzfeld, loc. cit. p. 556. |