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 disappeared. 
In the mammalia where the premaxillaries are generally well 
developed to support the incisor teeth, the prenasal cartilage is, as 
might be supposed, usually rudimentary 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 ( Ornilhorhyn- 
chus, Miniopterus ), 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. 
t Pv. Broom. "On the Homology of the Palatine Process of the Mamma- 
lian Premaxillary." Proc. Linn. Soc. N.S.W. (2nd Ser.) Vol. x. 1895. 
