226 DR. R. BROOM ON THE ORGAN OF []\Iar, 18, 



more anterior sections have been seen to form the septal turhinals, 

 become detached fi'om the inner plates. 



A section immediately behind the anterior end of the palatine 

 process shows the inner plate of the nasal-floor cartilage dipping 

 down in the cleft between the palatine process and the premaxilla, 

 supported on its inner side by the vertical plate of the former. 

 The papilla is fairly large, and the naso-palatine canal is seen 

 passing upwards and inwards by its side. 



A few sections beyond this plane we see (PI, XXI. figs. 5 & 6) 

 that the na -o-palatine caiisl {n.p.c.) on passing further upwards 

 turns outwards as it opens into the nasal cavity. On its upward 

 passage it receives the duct of Jacobson's organ (J.o.). This dvict, 

 which lies almost vertically, passes between the large vertical plate 

 and the apparently detached outer portion of the nasal-floor 

 cartilage and opens into Jacobson's organ near its anterior end. 

 The organ extends very slightly in front of the point where the 

 duct is given off. 



Immediately behind the duct (PI. XXI. fig. 7), the lower end 

 of the vertical plate becomes attached to the outer, apparently 

 detached, portion, forming a floor to the organ. It will thus be 

 seen that Jacobson's cai-tilage has an outer bar exactly as in 

 Marsupials. 



The Jacobson's organ itself is of moderate length and presents 

 no features of special interest in the foetus. The posterior part 

 of it aj)pears to be devoid of sensory epithelium and to be merely 

 the duct conveying the secretion from a large number of glands. 



In the adult, the cartilages ai-e essentially similar in arrange- 

 ment to those in the fcetvis, but an additional featvu-e is to be 

 observed in the presence of a weU-developed cartilage in the 

 papilla (PI. XXI. fig. 8). The organ is in section (PI. XXI, 

 fig. 9) somewhat kidney-shaped, with a single large vessel running 

 along the hilus. The sensory epithelium is confined to the imier 

 wall, and the organ is abundantly supplied with glands (g.). 



From the above description it will be seen that in its relations 

 the oi'gan has little or no resemblance to the highly specialized 

 type met with in most Eutherians, and that aU its peculiarities 

 are those typical of Marsupials. 



In the Marsupial the following may be regarded as the most 

 typical features of this region of the head: — (1) The Jacobson's 

 organ opens into the naso-palatine canal near the point whei'e 

 the canal opens into the nasal cavity ; (2) the anterior part of 

 Jacobson's organ is protected externally by a cartilaginous bar 

 which passes from the outer edge of the lower part of Jacobson's 

 cartilage behind to the outer edge of the upper pait in front ; 

 (3) the naso-palatine canal is never supported by a cartilaginous 

 process from the nasal-floor cartilage ; (4) the nasal floor has no 

 cartilaginous support behind the region of the naso-palatine canal ; 

 (5) the papilla has a well-developed cartilage ; and (6) the Jacob- 

 son's organ has usually a single large vessel running along its 

 outer face. 



