THE ANATOMY OF BIRDS. — 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 bh are in the second arch, and form respectively the ceratohj/al and hasihyal 

 bones ; the pieces cbr, ebr, and Vbr are in the third arch, and form respectively the eerato- 

 branchial, epibranchial and basibranchial bones. These pieces of the third arch have already 

 outgrown those of the second arch, and they wiU 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 trabeculee in frotrt 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, to ps and ak). This partition, besides forming, finally the mterorbital 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 

 t(i the optic foramen, 3, ending in the ante- 

 rior cUnoid wall, asc, separated from the 

 (parachordal) posterior clinoid wall by the 

 original pituitary space, now the opening 

 through which the carotid arteries, ie, 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- 



ael 



Pis. 66. — Head of a chick, second stage, after five days 

 of incubation, section in profile; X 6 diameters. cvl,cv2fCv3, 

 first, second, and third cerebral vesicles ; 1, place of the 

 first nerve, the olfactory ; 2, pla«e 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; tic, notochord; oc, occipital condyle, thence 

 to pet 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 ; pas and^ji reference lines are in the chick's mouth ; mJ 

 lates with the auditory sac and parts of meckelian cartilage (lower jaw) ; ch a,nd bh, oeratohyal and 



the exoccipital cartilage. The relations at ^"^""^^^ p^''' ""''^ ^^°'^ "^ '"""''^ *'"'«• '^^'^"^ ^^'^^«''' 

 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 wbicd). In fig. 67, mfc 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 cartUage, the handle of which is continuous with the 

 substance of the ear-capsule ; the sickle-shaped piece behind which is the tympanic wing <jf 



