Page:Encyclopædia Britannica, Ninth Edition, v. 3.djvu/721

orbito-uasal wall is one continuous plate of cartilage, seem ingly only a crest upgrowiug from the coalesced tract of the trabeculse. The arched wings of this part, which we saw in the first stage, can be seen to be marked off into three regions an aliethmoid (al.e.), an aliseptal (al.s.), and an alinasal (al.n.); in the latter is the external nostril, and dividing the eye from the nose is a flat partition, the pars plana (p.p.) Between the nasal openings and their cur tains and valves, the prenasal cartilage (p.n), still arcuate, is yet rising in front ; whilst, behind and above, the great middle wall (eth.) terminates by a bud of cartilage, which marks the fore-end of the cranial cavity, and the groove on each side below this is for the nerve of smell (1). The oval pituitary space has become a neat round opening, through which the internal carotid arteries enter ; it never choudrifies below, and has to be floored afterwards by secondary bone. The upper hyoid element (st.) has now freed itself from the periotic cartilage, bringing away so much as serves for the dilated dorsal end, and thus leaving an open window (fenestra ovalis), to which this forms the accurately fitted shutter. In front of this small cartilage, the quadrate (q.) shows almost its adult form, and the double lower condyle fits into a sinuous concavity on the end of Meckel s cartilage (ink.) This free bar the mandible sends backwards a posterior, and inwards an internal angular process. The later and feebler pterygo- palatine sickles are now formed internally of small thin- walled spheroidal, and externally of fusiform, cells of a larger size. They ossify before chondrification can take

place.

Interposed between the skin of the palate and fauces below and the basis cranii above, is a thick mat of granu lar tissue, which does not, however, chondrify, but ossifies a-> the parasphenoid and basi-temporal bones.

The FIG- 7. Skull of CMck, third stage,

part of basal region, from above, X 13 diameters, a.s, root of ali- sphenoid; b.s.. basi-sphenoid. The notochord (n.c.) is seen to lie in an open space in front the posterior basi-cranial fontanelle and to be bony behind ; the bony matter is the rudiment of the basi-occipital.

Cranium of Fowl—Third Stage.—After about three or four days, that is, about the middle of the second week of incubation, the chondro-cra- nium has not only undergone great changes in size and form, it is also now beginning to become an osteo-cranium. All that is cartilaginous has acquired a neat finish (figs. 7, 8); the occipital condyle (o.c.) is perfect; the super-occipital arch is complete (fig. 8, 5.0.); and bone is forming in three place.? in the occiput (n.c., e.o.), and the bone (pa.s.) which un derlies the orbital septum has grafted itself upon the carti lage of the basi-sphenoidal region. That bone, the para- sphenoid (r.b.s.), and the other investing bones, are now grow ing in the tissue between the skin and the cranio-facial ele- ments ; those that can be seen below are shown in figure 8, the remainder can be de scribed in riper stages. downward bend (not shown in the figure) of the prenasal rostrum (p.n.) is but gentle; this long spatulate continua tion of the base of the orbito- nasal septum, the " basi-trabecular bar," or foremost key stone piece of the body, is now at its fullest height of growth, and ready to decline. It is now the accurate counterpart of the axis of the cutwater of such Fishes as the Skate, Saw-fish, and Shark. Those who look for the underlying unity of the various types may here see how fit this unpaired rod is to have modelled on it all kinds of beaks of Birds. On this bar a tri-radiate patch of bone is formed right and left, leaving it as yet uncovered below, above, and at the fore-end. These are the young premaxillaries, and are bones that in most Birds, as in Osseous Fishes, over shadow and starve the upper jaw-bones, or maxillaries proper, so large, relatively, in most other types. Already they have each a palatine, a nasal, and a dentary process. The machinery of the first post-oral is shown (fig. 8), all save the free mandible, which will be described in a more advanced stage. The quadrate (q.) is ossifying; the ptery goid and palatines (pg., pa.) are ossified; they are very simple bars. Between the premaxillary and the quadrate are bones that have been formed in the outer part of the " maxillo-palatine process ".of the embryo; they are the feeble maxillary, with its ingrowing maxillo-palatine plate, and the still feebler jugal and quadrato-jugal (mx., mx.p., J-, 1-J-)

FIG. 8. Skull of Chick, third stage, under view, x 10 diameters. Around the

large prenasal cartilage p.n~, which, behind, runs into the septum nasi, are seen the premaxillnrier< (px.) These are the commencement of a chain of splints running to the quadrate (q.), viz., mx., maxillary; mx.p^ its inner process; j., jugal; q.j., quadrato-jugal. AVithin these are pa, palatine; pg., pterygoid; r.bt., rostrum of basi-sphenoid (parasphenoid) ; this is spreading into the basi- sphcnoidal region, flanked by the lingulce. and above and behind are the ali- sphenoids (a j.) In the broad part are the following, viz., b.t., basi-temporals ; ty., tympanic cavity ; f.m., foramen magnum; b.e.o., the bony exoccipitals ; j.o., super-occipital* Other letters as above.

On the mid-line a grooved style of bone, the rostrum of the ]; arasphenoid, carries the rounded lower edge of the