556 MAMMALIAN PRENASAL CARTILAGE, 



skull of an embryo chick of the middle of the second week as 

 figured by Parker,"^ the prenasal is seen as a well-developed 

 median cartilage, passing to the front of the beak and separating 

 the two premaxillaries from each other. In the chick two days 

 old the premaxillaries have united and quite obliterated the pre- 

 nasal in front, reducing it to a small median spur extending in 

 front of the nasal septum and lying on the palatal surface of the 

 hinder part of the united premaxillaries. In the old bird the 

 cartilage has quite disa]3peared. 



In the mammalia where the premaxillaries are generally well 

 developed to support the incisor teeth, the prenasal cartilage is, as 

 might be supposed, usually rudimentar}^ or absent. There is 

 moreover in most mammals another peculiarity unfavourable to 

 the existence of the prenasal — the union of the prevomer with 

 the premaxillary. As I have recently shown, f the mammalian 

 prevomer, though occasionally a distinct element ( Ornithorhyn- 

 chus, Afi7iio2?terus), usually early anchyloses with the premaxillary 

 or becomes ossified in connection with it. As this structure lies 

 below the nasal septum, in being connected anteriorly with the 

 premaxillary, the anterior palatal region becomes to a large extent 

 shut off from the nasal septum, and in the adult condition where 

 the bones are closely articulated in the middle line completely so. 

 In a few interesting instances, however, the prenasal element has 

 succeeded in asserting itself. 



The most remarkable developments of the prenasal are in the 

 egg-laying mammals — Ornithorhynchus and Echidna. The pre- 

 maxillaries in both these forms are edentulous and feebly developed, 

 and in neither are they provided with palatine processes, as in 

 Ornithorhynchus the prevomer is distinct and in Echidna quite 

 absent. 



* W. K. Parker. On the Structure and Development of the Skull of the 

 Common Fowl. Phil. Trans. 1869, p. 755. 



+ R. Broom. "On the Homology of the Palatine Process of the Manmia- 

 lian Premaxillary." Proc. Linn. Soc. N.S.W. (2nd Ser.) Vol. x. 1895. 



