hatching, the skull of the chicken is excellent for com parison; as iu Cattle, so in the Fowl and its relations, " then- young ones are in good liking " as soon as they appear ; hence" they are strong in bone and sinew very early, the
growth-processes taking place rapidly.The under view (fig. 13) shows, behind the orbits, a strong- cranial box. Synchondroses of the remainder of the chondro- cranium divide the endo-skeletal bone-territories ; and that organic attraction which causes the perfect correlation of the shallow with the deeper layers of the mesoblast, has given us here bony encasements, that not only fill in what was wanting in the chondro-cranium, but also overlie and double the strength of the ossified cartilage itself. All is neat and finished now, and now is the nick of time in which can be seen most of the sutures, so soon to be obliterated by the almost universal ankylosis that takes place afterwards a process by which Nature, in the Bird, rapidly fills in nearly all her footprints. Here she is as hard to bind as Proteus himself ; and the morphological worker is often sore bestead to catch all the transformations. If the chick were to retain throughout life its present form of face, it would much resemble a Hemipod. We have seen how its skull becomes metamorphosed from a lower into a higher and a still higher type day by day.
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FIG. 14. Horizontal section of skull-base of same Chick, X 4 diameters, a.t.r..
anterior tympanic recess; if, stapes; 7, portio mollis passage (meatus in terims).
This basal view shows that the bony pieces of the occipital arch are fast forming a strong ring, after the manner of a vertebra. The keystone piece (s.o.) is now one bone, with the remains of the suture above. The notochord still exists in the basal piece (6.0.), and is seen in the condyle, the dimple of which is caused by it. The basi-temporal plate (b.t.) is now a low triangle, with its base behind ; the base is, as it were, gnawed, the jagged edge joining the overlying spheno-occipital by a squamous suture. This thick plate is emarginate in front, below the meeting of the eustachian tubes (eu.), and this notch is the mark of the original symmetry of this double bone. Grooved gently at the mid-line, the basi-temporal is mammillate on each side, these elegant swellings being due to the cochleae that are encysted in the ossifying cartilage above. Behind, it is eared, and over these ears the internal carotids creep as they seek the pituitary space ready to form the "circle of Willis." The bone above and in front of the basi-temporal is a compound of the rostral parasphenoid and the proper basi-sphe- noid. Altogether, this is a very extensive and multi form element. -Directly in front of the basi-temporal it is scooped at the meeting of the eustachian tubes - diverticula of the first visceral cleft. In front of that part the rostrum is soon a free structure, grooved for the inter- orbital septum ; but behind and above the basi-temporal the bony mass is all one, and it has an upper and a lower "wing on each side behind the strong wall which it has built around the pituitary well. The upper of these wings are ossifications of that cartilage which we saw was formed by coalescence of the sub-apical part of the trabeculte with the fore-end of the parachordal plates.
The large posterior basi-cranial fontanelle has now be come the deep chink which exists as a ditch between these post-pituitary banks (fig. 14, p.b.f.) On the upper surface the chink reaches the basi-occipital ; in the horizontal section (fig. 14) it is cut away behind.
The lower and outer pair of wings of the basi-spheuoid are very large (figs. 13 and 14); they build, on each side, the anterior tympanic recess (a.t.r.], and their starting point of growth is from the free apices of the trabecuke, which are thus feathered with these large coiled lamina? of periosteal bone that enclose another diverticulum of the first visceral cleft, which lies over the eustachiau passage.
Close in front of the eustachian groove the soldered part of the rostrum widens into a pair of projections, and, upon these, obliquely placed facets of cartilage are attached for articula tion with counterpart cartilages on the pterygoid bones (py.)
These perfectly distinct cartilages are the result of a peculiar metamorphosis of the outstanding (basi-pterygoid) spurs of the trabecuke. In the Ratitce, as in Lizards and Serpents on the one hand and Mammals on the other, these external pterygoid plates or processes are a direct out growth of cartilage, the posterior conjugational spurs that grow out for union with the ptery go-palatine arcade.[1] Like the subdivision of the ethmo-trabecular wail, this segmentation of originally continuous cartilage is of the highest morphological importance. Several other things of this kind will be found in this class, where the Vertebrate pattern has been specialized and metamorphosed to its highest degree, as if to produce types that should be as imagines in relation to the forms beneath them. The remainder of the rostrum just runs, pointed, to the edge of the cranio-facial cleft (fig. 16, r.b.s. to a.p.) In front of this notch the septum nasi is seen, narrow and rounded, and it terminates below in the starving prenasal (s.n.,>.n.) Around and in front of that rod the now single premaxillary clings; it has its under surface grooved to the end, where that rod lay, its sides developed into the sharp dentary region (fig. 13,c/.^a-.),aud its under part growing backwards as two nearly parallel bands, the palatine processes (p.px.}, that articulate with the palatine bones (pa.) These latter bones run backwards in the same gently diverging manner, and then curve inwards to be tied the one to the other, before they bend outward again, foot-like, to articulate with the pterygoids ( py. ) Theso palatines are very simple ; they give off from their main bars merely a scooped lamina, growing towards the skull base, becoming the ethmo-palatine in front; and this part articu lates with a splint of the trabecular arch, the small styli- form vomer (v.), which is notched behind, bluntly pointed in front, and primarily azygous.
The pterygoids (py.} are stout little bony mallets, with a pad, as if of leather the cartilaginous articular meniscus. Where each bone glides on the similarly padded trabecular region these facets look upwards and inwards. The end of the palatine is articulated by strong fibres to a sinuous notch on the fore-end of the pterygoid. This notch is bounded by the basi-pterygoid facet below, and by a stunted " meso-pterygoid process " above, which rides over the palatine. It is arrested in the Gallinaceoe, and never becomes segmented off (see below). The hinder end of the pterygoid is scooped below to articulate with the counterpart knob on the front face of the quadrate. A joint cavity is formed here. Above that joint, and looking upwards and forwards, is a very stunted " epi-pterygoiu " process, which clings to, and is strongly strapped upon, the quadrate by fibrous tissue.
- ↑ See Phil. Trans. 1866, plate 7, fig, 4, a.p. ; and ibid., 1874, plate 34, fig. 2, e.pg.
