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) 
which 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 IL (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, which 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; (b) in the otie region: proGtic, epiotic, and opisthotic; 
