152 



GENEBAL OliXITIIOL G Y. 



nasal plate, which is so arched over downward as to disapjiear from this view, as seen in 

 fig. 65, where fn is the fronto-nasal process, and n is the future external nostril. After 

 uniting in the inter-nasal plate, the fore ends of the trabeciiUe se2)arate and become free ; their 

 free ends are the under extremities of this first visceral arch (first and only pre-oral arch). 



Tlie same chick's head, now viewed from below, fig. 05, shows the squarish aperture, vi, 

 of the future mouth ; the three post-oral arches, with their respective cartilaginous bars, out 

 of which are to be formed the bones of the jaws and tongue. 1, 2, 3, are the coiTCsponding 

 visceral clefts, between the arches ; the first of these is to be modelled into the ear- 

 l)assages (outer and middle ear and eustachian tube) ; the others will disajipear. The quadrate 

 cartilage, g, is the same that was seen in fig. ()4 ; it is already nearly in position, between the 

 hind ends of the scaflblding of the upper and under jaw. The curved subocular or maxillo- 

 palatine bar, mxp, developed in the first post-oral arch, already indicates anteriorly palatine, 

 pa, and posteriorly, jiterygoid, pg, parts ; it will form the bones so named, and others of tlie 



Fig. 64. — Skull of chick, fifth day of incubation, 

 X9 diameters. Seen from above, the membranous roof 

 of the skull and the brain removed, cvl, ;interior cere- 

 bral vesicle ; c, eye ; c, notochord, running through the 

 middle of the basilar plate or parachordal cartilage, in 

 which are already visible the rudimentary ear-parts, cl. 

 the cochlea, hsc, the horizontal semicircular canal ; pts, 

 the pituitary space, bounded by 1r, the trabecule, 

 vrhich come together before it to form the fronto-nasal 

 plate, fn, in fig. 65; U/, Ihtgnh. or bridge connecting 

 trabecute with parachordal cartilage ; 5, notch after- 

 ward becoming foramen ovale for passage of parts of 

 the fifth (trifacial) nerve ; 9. foramen for hypoglossal 

 nerve ; q, separate cartilage forming the future quad- 

 rate bone. (After Parker, in Encij. Brit.) 



Fig. 65. — Same as fig. 64, but seen from below. 

 cfl, anterior cerebral vesicle; e, eye; m, month: pfs, 

 pituitary space;//;, fronto-nasal plate; tr, ends of the 

 trabeculse, free again after their union and bent strong- 

 ly from the original axis of the trabeculse ; h, exter- 

 nal nostril ; mxp, subocular bar of cartilage, or ptery- 

 go-palatine rod, to form pa, palatine, and prj, pterygoid 

 bone, and other parts of the upper jaw, as the maxil- 

 lary, jugal and quadrato-jugal; rj, quadrate cartilage, 

 same as seen in fig. 64 ; ml; meckelian cartilage, to form 

 lower jaw ; these parts are in the first post-oral visceral 

 arch; ch, cerato-hyal, and bJi, basihyal, of second post- 

 oral arch; cbr, cerato-branchial, ebr, epi-brauchial, 

 66r, basi-branchial, of third post-oral arch; the :)arts 

 of the second and third arch all going into the yoid 

 bone. 1, 2, 3, 1st, 2d, 3d visceral clefts, whereof i,ue 1st 

 is to be modified into the ear-passages, and the others 

 are to be obliterated. (After Parker.) 



upper jaw. This subocular bar is an antero-superior part of the fii'st post-oral arch, of which 

 q and ink are a postero-inferior portion ; the cleft of the future mouth is to lie between them. 

 The lower jaw bone, or mandible, is entirely developed from mk, its several bones developing 

 around this rod of cartilage, the meckelian cartilage ; it is to become movably articulated with 

 the bone, the quadrate, into which q will be transformed. Thus the postero-inferior part of 

 the first post-oral arch (second of the whole series of arches) begins in two pieces, one of which 

 is to become the suspensorium, or suspender of the mandible, and the other the mandible 



