THE ANATOMY OF BIIWS. ~ OSTEOLOGY. 



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itself. The rest (jf 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 tlie second arch, and form respectively the ceratohijal and hasihyal 

 bones ; the pieces cbr, ebr, and hbr are in the third arch, and form respectively the cerato- 

 branchial, epibrancliial and basibr<mchial bones. These pieces of the third arch have already 

 outgrown those of the secoml arch, and they will form the greatest part of tlie 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 unconfonuably, the latter rising high between 

 second and third cerebral vesicles to form the pcjsterior pituitary wall, pel, in which the axial 

 skeleton properly ends. There are other changes in the parachordal cartilages. The inter- 

 nasal plate, formed by th(! unitju of the trabecute iu front of the pituitary space, has become a 

 vertical median wall between the (dfactcjry and optic chambers of the right and left sides (^jw 

 and eth, to lis and ale). This partition, besides forming finally tlie interorbital septum which 

 divides the right and left orbits, will undergo further notable changes in direction, and will 

 ilevelop 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, nliseptal, and alinasal. 

 This plate extemls backward iu 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. p)renasal carti- 

 lage, pn, to become the axis of the beak. 

 The mouth is become already better formed, 

 the axis of its cavity pointing more fijrward 

 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 nmch of 

 tlieir true form. The quadrate develops 

 an orbital process, which extends free into 



the orbit, and an o<ic rn-ocess which articn- . , „ ,. ■ ^ ,i,. i, , 



' ^ region ; pa 'dntlpr/ reierence lines are in tlie cbick s iiioutli ; mi: 



lates with the auditory sac and parts of meckelian cartilage (lower jaw); c/i and 6/;, ceratoliyal and 

 the exoccipital cartilage. The relatiou.s at ^^^'^^^^ ^^'^^ °*'*^" ^>'"*'^ "^^ ^^^-^^" ^°""- '^^^^"^ ^^^^"■'■^ 

 this stage have not hecu made out in the fowl, but are figured and described from the corre- 

 vSpouding stage of the Eurrtpean house martin (Chelidon tirbica). In fig. 67, m^■ is the cut 

 stump of the meckelian cartilage, of which ar is the articular part; q is the quadrate, of which 

 a bacli:ward process is seen articulating with teOj the tympanic wing of the exoccipital. Just 

 below and beliind this otic, process of the fjuadrate, 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 tniwel-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 -jJ 



cicl 



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

 of incubation, section in profile ; x diameters, cvl, ci'2, c?'3, 

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

 first nerve, the 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 f)Osterior, pcU 

 clinoid walls; ?ic, notochord; oc, occipital condyle, thence 

 to pel being the original parachordal cartilage, here seen in 

 X>rofile; t?o, exoccipital; e^A, ethmoid, with pn, its presphe- 

 noid region posteriorly, and 7>h, jire-nasal part; this whole 

 plate afterward developing into parts of the nose and the 

 partition between the eyes; pa, palatine; pg, pterygoid 



