432 THE DEVELOPMENT OF THE CHICK 



to the definitive lower jaw of selachians. It consists of two 

 rods of cartilage in the rami of the mandibular arch, which articu- 

 late proximally with the processus articularis of the palato- 

 quadrate cartilage, and meet distally at the symphysis of the 

 lower jaw. The form of the articulation of the lower jaw is early 

 defined in the cartilage (seven to eight days). 



Hyoid Arch. The skeletal elements of the hyoid arch consist of 

 proximal and distal pieces (with reference to the neurocranium) 

 w^hich have no connection at any time. The former are destined to 

 form the columella, and the latter parts of the hyoid apparatus. 

 The columella apparently includes two elements (in Tinnunculus 

 according to Suschkin, quoted from Gaupp) : a dorsal element, 

 interpreted as hyomandibular, in contact with the wall of the 

 otic capsule, and a small element (stylohyal) beneath the former. 

 The two elements fuse to form the columella, the upper end of 

 which is shown in Fig. 168. The stapedial plate (operculum of 

 the columella) is stated to arise in Tinnunculus from the wall 

 of the otic capsule, being cut out by circular cartilage resorption 

 and fused to the columella. 



The distal elements of the hyoid arch consist of (1) a pair 

 of ceratohyals, which subsequently fuse in the middle line to 

 form the entoglossal cartilage, the proximal ends remaining free as 

 the lesser cornua of the hyoid, and (2) a median unpaired piece 

 (copula I or basihyal) behind the united ceratohyals (Fig. 245). 



First Branchial Arch. The skeletal elements of the third visceral 

 (first branchial) arch are much more extensive than those of the 

 hyoid arch. They are laid down as paired cerato- and epi-branchial 

 cartilages on each side, and an unpaired copula II (basibranchial I) 

 in the floor of the pharynx, in the angle of the other elements 

 (Fig. 245). The cerato- and epibranchials increase greatly in 

 length, and form the long curved elements (greater cornua) of the 

 hyoid, w^hich attain an extraordinary development in many birds. 



Ossification of the Skull. The bones of the skull are of two 

 kinds as to origin: (1) those that arise in the primordial cranium, 

 and thus replace cartilage (cartilage bones or replacement bones), 

 and (2) those that arise by direct ossification of membrane (mem- 

 brane or covering bones). 



The cartilage bones of the bird's skull are: (a) in the occipital 

 region; the basioccipital, two exoccipitals, and the supraocci- 

 pitals; (6) in the otic region: procitic, epiotic, and opisthotic; 



