OEGAN OF JA.COBSON IN SPnENODON. 415 



detailed account of the relations of the organ, and to confirm 

 Seydel's opinion as to its mammalian afBnities. The sections 

 which most satisfactorily show the relations of the organ are a 

 series of transverse sections of the head of an embryo of Dendy's 

 stage E. At this stage the bones of the head are well ossified, 

 and the organs in young animals have their relations in a more 

 satisfactory condition for enabling comparisons to be made than 

 at later stages, when the primitive structure becomes frequently 

 somewhat obscured by specialization. 



lu a transverse section through the premaxillaries, the bones 

 are seen to be separated by a feebly developed portion of the 

 internasal cartilaginous septum. A similar condition is met 

 with in Echidna, but is not present either in the lizard, snake, 

 or tortoise. 



Immediately behind this plane, the nasal septum is found to 

 be greatly expanded in its lower half as seen in fig. 1 (Pi. 41). 

 It is continued externally into the delicate little cartilage that 

 supports the external nasal opening. 



A short distance behind the plane illustrated by fig. 1, the 

 lower part of the median cartilage is found to have four 

 openings — two large ones above for the accommodation of the 

 anterior parts of Jacobson's organ, and two smaller ones for 

 the anterior ends of the prevomers. Figs. 2 & 3 illustrate 

 sections in this region, and fig. 3 shows the lower part of the 

 cartilage forming a cartilaginous support for the papilla. The 

 presence of a papillary cartilage is unknow^n in Lizards or Snakes, 

 nor has it, so far as I am aware, been found in Chelouians or 

 Birds, but it is met with in the Crocodile. Among Mammals a 

 papillary cartilage has been shown by Wilson to be present in 

 the foetal Echidna ; and I have shown that a well-develojjed 

 cartilage is present in the majority of Marsupials, and among 

 higher forms in Macroscelides and Miniopferus. 



In figs. 2 & 3 is seen the moderately well-developed septo- 

 maxillary bone. It forms the floor of the anterior part of the 

 nasal cavity, and to some extent protects the roof of Jacobson's 

 organ. In its relations it is seen to be strikingly dissimilar to 

 the septo-maxillary of either the lizard or the snake. In the 

 Squamata the organ of Jacobson is never roofed by cartilage as 

 in Sphenodon, and the septo-maxillary bone is highly developed 

 to protect the very large organ. The bone is firmly attached to 

 the nasal septum and usually surrounds the upper part of the 



30* 



