THE ANATOMY OF BIBDS. — OSTEOLOGY. 
153 
itself. The rest of the pieces belong to the second and third post-oral arches, and all 
together make up the very composite hyoid bone, or bone of the tongue (figs. 72, 73, 74). The 
pieces ch and hh are in the second arch, and form respectively the ceratohyal and hasihyal 
bones ; the pieces cbr, ehr, and bbr are in the third arch, and form respectively the cerato- 
hranchialj ejnhranchial and basibranchial bones. These pieces of the third arch have already 
outgrown those of the second arch, and they will form the greatest part of the hyoid bone. 
In the second stage, after the fifth day of incubation, but before any ossification has 
begun, a vertical section shows the appearances represented in fig. 66. The parachordal and 
trabecular cartilages are applied to each other unconformably, the latter rising high between 
second and third cerebral vesicles to form the posterior pituitary wall, pel, in which the axial 
skeleton properly ends. There are other changes in the parachordal cartilages. The inter- 
nasal plate, formed by the union of the trabeculse in front of the pituitary space, has become a 
vertical median wall between the olfactory and optic chambers of the right and left sides (pn 
and eth, tops and ale). This partition, besides forming finally the interorbital septum which 
divides the right and left orbits, will undergo further notable changes in direction, and will 
develop lateral plates and processes, which 
will make up the nasal labyrinth and the 
partition between the cavity of the nose 
and that of the eye, when any exists. Such 
lateral developments of the ethmoid plate 
are the aliethmoid, aliseptal, and alinasal. 
This plate extends backward in mid-line 
to the optic foramen, 2, ending in the ante- 
rior clinoid wall, asc, separated from the 
(parachordal) posterior clinoid wall by the 
original pituitary space, now the opening 
through which the carotid arteries, ic, enter 
the brain cavity. Besides ethmoidal parts 
proper, the plate develops at what will be 
the end of the upper beak a prenasal carti- 
lage, pn, to become the axis of the beak. 
The mouth is become already better formed, 
the axis of its cavity pointing more forward 
than downward; and great changes are 
undergoing in parts of the ear at the back 
corner of the mouth. The quadrate and 
meckelian cartilages are assuming much of 
their true form. The quadrate develops 
an orbital process, which extends free into 
the orbit, and an otic process which articu- 
lates with the auditory sac and parts of 
the exoccipital cartilage. The relations at 
this stage have not been made out in the fowl, but are figured and described from the corre- 
sponding stage of the European house martin (Chelidon urbica). In fig. 67, mJc is the cut 
stump of the meckelian cartilage, of which ar is the articular part ; q is the quadrate, of which 
a backward process is seen articulating with teo, the tympanic wing of the exoccipital. Just 
below and behind this otic process of the quadrate, exactly where in riper embryos is the 
fenestra ovalis in which is fitted the foot of the stapes or stirrup-bone of the middle ear, there 
appears a trowel-shaped projection of cartilage, the handle of which is continuous with the 
substance of the ear-capsule ; the sickle-shaped piece behind which is the tympanic wing of 
Fig. 66. — Head of a chick, second stage, after five days 
of incubation, section in profile ; x 6 diameters, cvl, cv2, cv3, 
first, second, and third cerebral vesicles; 1, place of tlie 
first nerve, tlie olfactory ; 2, place of second nerve, the 
optic ; ic, internal carotid artery, running into skull at what 
was originally the pituitary space, now an opening bounded 
in front by the anterior, acl, behind by the posterior, pel, 
clinoid walls; nc, notochord; oc, occipital condyle, thence 
to pel being the original parachordal cartilage, here seen in 
profile ; eo, exoccipital ; eth, ethmoid, with ps, its presphe- 
noid region posteriorly, and pn, pre-nasal part ; this whole 
plate afterward developing into parts of the nose and the 
partition between the eyes; pa, palatine; pg, pterygoid 
region; J9a and^^ reference lines are in the chick's mouth ; mJc 
meckelian cartilage (lower jaw); ch and bh, ceratohyal and 
basihyal parts of the hyoid or tongue bone. (After Parker.) 
