226 



GENERAL ORNITHOLOGY 



j>s. 



the ceraiohyal and basihyal bones ; the pieces chr, eh; and hbr are in 

 the third arch, and form respectively the ceratobranchial, epibranchial, 

 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 hyoicl 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 Avail, p<^l, in 

 Avhich the axial skele- 

 ton properly ends. 

 There are other 

 changes in the para- 

 chordal cartilages. 

 The internasal plate, 

 formed by the union 

 of the trabeculse in 

 front of the pituitary 

 space, has become a 



Fio. 66.-Head of a chick, second stage, after five days of -,.„i.- _i ,-,,p.i;<,„ ^lU 

 incubation, section in profile ; X 6 diameters, crl, cv2, c(;3, first, vextiud,! iiieuitiii wa.ii 

 second, and third cerehral vesicles ; 1, place of the first nerve, j^gt-yyeen the olfactorv 

 the olfactory ; 2, place of second nerve, the optic ; ic, internal _ ' •' 



carotid artery, running into skull at what was originally the and OptlC chambers 

 jiituitary space, now an opening bounded in front by the an- 

 terior, ad, behind by the posterior, pel, clinoid walls ; tic, noto- 

 chord ; oc, occipital condyle, thence to jxl being the original 

 parachordal cartilage, here seen in profile ; eo, exoccipital ; eth, 

 ethmoid, with ps, its presphenoid region posteriorly, and pn, 

 prenasal part ; this whole plate afterward developing into parts 

 of the nose and the partition between the eyes ; jm, palatine ; 

 pg, pterygoid region ; })« and pgr reference lines are in the 

 chick's mouth ; mk, Jleckelian cartilage (lower jaw) ; c/t and bh, 

 ceratohyal and basihyal parts of the hyoid or tongue bone. inteVOTbital SCVtUltl 

 (After Parker.) .' ^ 



which divides the 

 right and left orbits, will undergo further notable changes 

 in direction, and will develop lateral plates and processes, which 

 Avill 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 ullnasal. This plate extends backward in mid-line to the optic 

 foramen, 2, ending in the anterior clinoid wall, acl, separated from the 

 (parachordal) iwsterior clinoid, jkI, 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 



cccl 



of the right and left 

 sides (j)7i and eth, to 

 ps and ale). This 

 partition, besides 

 forming finally the 



