

VIII.] THE MANDIBULAR ARCH. 243 



The second arch (hyoid), with its contained bar, 

 though retaining in some forms (Elasmobranchii) its 

 branchial function, has in most acquired additional 

 functions, and has undergone in consequence various 

 peculiar modifications. 



The succeeding arches and their contained bars 

 retain their branchial function in Pisces and some 

 Amphibia, but are secondarily modified and largely 

 aborted in the abranchiate forms. 



The ordinary visceral arches in the chick are, as we 

 have seen, sufficiently obvious, while as yet their meso- 

 blast is quite undifferentiated ; but in the three ante- 

 rior of them rods of cartilage are subsequently deve- 

 loped and begin to make their appearance about the 

 fifth day. 



The first arch (mandibular), it will be remembered, 

 budded off a process called the superior maxillary pro- 

 cess. The whole arch, therefore, comes to consist of 

 two parts, viz. a superior and an inferior maxillary pro- 

 cess ; it is in the latter of these that the cartilaginous rod 

 on each side is developed. The membranous tissue in the 

 superior maxillary process is called, from its subsequent 

 fate, the ptery go-palatine bar, and is in the chick ossified 

 directly without the intervention of cartilage. In the 

 inferior maxillary process two developments of cartilage 

 take place, a proximal and a distal. The proximal 

 cartilage is situated (Figs. 76 and 79, q.) at the side 

 of the periotic capsule, but is not united with it. It is 

 known as the quadrate, and in the early stage is merely 

 a small knob of cartilage. The quadrate cartilage os- 

 sifies as the quadrate bone, and supplies the permanent 

 articulation for the lower jaw. The distal rod is called 



162 



