does not develop any obvious "middle turbinal " us in
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FJG n._Another section from the same Chick, further back, X 8 diameters, i.tb., inferior turbinal; al.s.. nlis<;pt;U ; n., nasal bone.
The differentiation cf distinct morpholo gical regions, which did not become obvious ia the membrano-cran ium, can now be seen to some extent in the well- developed cartilage. As in the Ratitce, the orbito-nasal septum was, Ill the Second Stage, a COntillUOUS sheet of Cartilage.[1]
Looking upon the trabeculae as the first facial arch> correlated to and supplied by the orbito-nasal nerve, we see why there should be a segmentation of this uppermost and foremost part of the face into a suspensorium and a free arch, such as is seen next behind in the mandible.
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ll<3 12 -- Axill! Pi t of the same skull, X 4
jiayetl mia- diameters. The new letters are n.n., groove ^ 01 nasa nerve; c.f.c., craiiio-facial cleft; ? IP *? -, supe;--vomerine lamina; p.e.. perpen- dicular ethmoid; i.o.f., inter-orbital fen-
cstra; s -> orbito-sphenoid.This segmentation has commenced, and, oddly enough, that part of the trabecular commissure which will be absorbed lias not lost its original flap shape (see fig. 1 2,-p.c- c.f.c., s.v.l.) The hour glass - shaped window here formed by meta- morphic action will be come a notch in a day or two, and the flat region of the trabeculse will be absorbed. Thus we get the Tinamine stage;[2] for in that inter mediate form between the Ratite and Carinate types the Tinamou the metamorphic pro- ppi^pq ni -P dtmrprl vmVl Way. and the bird is a natlVC, SO to speak, OI theStruthio-Gallinaceous "marches." Beneath the retral ethmoidal spike is seen the olfactory groove (1.); whilst in front of the cranio-facial fenestra (c.f.c.), the orbito-nasal nerve (n.n.) grooves the septum, gets bridged over by it, and creeps down to its own proper facial bar ths trabeculre. And the result of the metamorphosis in this Carinate bird is the formation of a pear-shaped fenostra (i.o.f.) between the eyeballs. This window was not cut out by Nature in a fit of economy (as the mere teleologists vainly speak), but is a fairly com menced separation of the common crest of the coalesced trabeculai from the ethmoid forwards and the presphenoid behind (p.e., p.s.)
The notch behind the cartilaginous frame of this window is formed by membrane into the optic foramen. A line drawn horizontally along the base of the fenestra, and another vertically to the upper margin of its narrow end, would mark out the starved presphenoid, without alee, of this bird ; for the mesethmoid and the basi-sphenoid (p.e., b.s.) meet below the fenestra. Underneath the inter-orbital plate the parasphenoid balk (quite distinct) is seen, and underneath the basal bones the basi-temporal slab (b.t.) Turning again to the bird s-eye view (fig. 9.), we see that the notochord is now enclosed in a spearhead-shaped bone, the basi-occipital (h.o.) It is ridged above by the enclosed notochord, and, behind, the swelling halves of the condyle (o.c.) are seen. This basal bone is truncated in front, and forms the hinder margin of the wide rounded " pos terior basi-cranial fontanelle" (p-b.f.) On each side are the exoccipitals (e.o.), and, above, the perfected occipital arch has a pair of super-occipitals (s.o.) as in man. The outer occipital region is grooved to receive the investing bones, and has tympanic wings to enlarge and protect that cavity. On each side of the fore-part of the basi-occipital is seen the cochlea (cl.) ; and in the ledge above the main periotic bone, the prootic (pro.) is largely spreading (the two lesser ossicles are appearing, but will be better shown in the next stage). The foramen ovale (5) is very large ; it is bounded behind by the prootic, and in front by the alisphenoid (a.s.) This " great wing" has a large central fenestra, round which the bone has crept. In somewhat younger specimens this bony matter was in two patches, one above and another below the fenestra. The same thing may be seen in arboreal birds, as the Common Sparrow.
The stem of the alisphenoid almost meets its fellow- process over the posterior clinoid wall (p.d.) In the stem is seen a " foramen rotundum." Outside, the ali sphenoid has a thick, secondary, post-frontal (sphenotic) wing (p.f.) The round pituitary space is walled in with bone. Its secondary parasphenoidal floor has been removed. We reserve a description of the fast-growing roof bones for the next stage, when they are most instructive for comparison with those of Fishes and Reptiles.
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FIG. 13. Ripe Chick s head, 1J inch long; lower view, X 3 diumefei-s. The
additional letters are v., vomer: /., lachrymal ; /., frontal ; eu., custachian opeii- ing; prf., posterior pterygoiJ processes ; sq., squamosal
