Page:Encyclopædia Britannica, Ninth Edition, v. 3.djvu/719

] pletely unite into a broad inter-nasal plate, which is bent over upon itself so as to appear on the under surface of the face. FIG. 2. The same as fig. 1, seen from below, /.n., fronto-nasal p^ate; n.. ex ternal nostril; m., mouth; mx.p., maxillo-palatine, containing pterygo- palatine bar (pg. pa.); mk., MeckeJ s rod, or free mandibular bar; cA, cerato- hyal; 6.A., basi-hyal; c.br. cerato-branchial; t.br., epi-branchial; ft.&r., basi- branchial ; 1, 2, 3, 1st, 2d, and 3d visceral clefts. The inter-nasal plate is arched and winged the rudimentary condition of the nasal sacs, the apertures of which are seen beneath the arched part below (fig. 2, n.) The trabeculre, antero-iuferiorly, become free again ; these free extremities are the ventral ends of this the first visceral arch ; they form the pith of the flat, emarginate rudimentary neb. This part is called the &quot;fronto-nasal process&quot; (fig. 2, /..) On the sides of the face behind the mouth are seen slits (fig. 2, 1, 2, 3) ; these are the &quot; visceral clefts,&quot; which are always developed in embryonic vertebrata. Between these clefts the tissue of the &quot; mesoblast &quot; is thickening into cartilaginous rods ; these rods are the post-oral vis ceral arches, the rudiments of the lower jaw and hyoid bone. But beneath the eye is seen an arcuate bar of tissue more solid than its surroundings ; this is the pterygo- palatine rod. It is developed in the maxillo-palatine pro cess of the mandibular or first post-oral arch ; it represents the bar of cartilage which, in the Frog, connects the man dibular suspensorium with the ethmoidal region. The first visceral arch, then, runs along in front of the parachordal cartilages ; the trabeculse cranii are its right and left moieties. The second arch can only be under stood by reference to the development of the lower types of Vertebrata; for in the Shark, Skate, Newt, and Frog the pterygo-palatine portion is not a distinct cartilage, but runs forward as a process of the suspensory part of the lower jaw. But this early division of a visceral arch into an antero-superior and a postero-inferior bar is very constant in the next or second visceral arch from the Skate up to Man. Let it be remembered that the cartilaginous stage is the second; in the first stage the skeletal parts are membranous. They are formed of soft stellate cells in the mesoblast. At present the subocular pterygo-palatine bar is very little denser than the tissue in which it is imbedded, but the mandible itself is fast passing into cartilage. Already it is in two pieces, a suspensory piece (figs. 1 and 2, &amp;lt;?.), and a free rod (mk.); the upper piece becomes the quad rate, and the lower, longer part the artieulo-Meckelian bar. The processes of the tuberous upper piece are indistinct, but it is sinuous below where it articulates with the thick end of the free bar. These bars nearly, but not quite, meet in the rudimentary chin, the ventral end of the lower jaw. Properly speaking, the hyoid arch is composed of two visceral arches ; but the term is now applied strictly to the first of these, namely, the second visceral, or the arch of the tongue. At present this arch has, with the one in front of it, its antero-superior piece quite soft ; it, too, is late in its development. The two lower pieces (fig. 2, c.h.) form the skeleton of the tongue. They are the cerato- hyals, and between these is a small basi-hyal (b.h.) answer ing to the first basi-branchial of a Fish. The third post-oral arch is very similar, but it is larger, and its upper piece is already fast chondrifying. That corresponds to the first epi-branchial of a Fish, the lower piece to the first cerato- branchial, and the median wedge to the second basi- branchial. FIG. 3. Head of Chick, second stage, after five days of incubation ; leneth of head, 4 lines ; X 6 diameters; a vertical section, c.r.l, c.v.2, c r.3, 1st, 2d, and 3d cerebral vesicles; 1, 2, localities of first and second nerves (olf acton and optic); pn., prenasal cartilage; tth^ ethmoid; a.cl., anterior clinoid wail ; p.el., posterior clinoid wall; n.c., notochord; o.c., occipital condyle (from this point to p.cJ. the cartilage is parachordal); e.o., exoccipital; i.e., internal carotid artery; pa^ palatine; pg., pterygoid region; mk.. Meckel s cartilage; cJt., cerato-hyal; b.h., basi-hyal; p.s^ presphenoidal region.

Cranium of Fowl—Second Stage.—In from twenty-four to thirty hours, or about the beginning of the sixth day of incubation, the chondro-cranium of the chick has under gone sundry and notable changes. A sectional view (fig. 3) shows that the hinder and front cartilages, parachordal and trabecular, are applied to each other unconfonnably, the parachordal tracts rising high between the second and third cerebral vesicles, and forming the posterior pituitary wall, a shelving structure in which the axial skeleton ends. A bird s-eye view of the hinder skull-floor at this stage (fig. 4) shows that the dorsal or hinder ends of the trabe culse have opened out, like a pair of callipers, and that the out-turned ends of the parachordal cartilages are fused with the inner margin of these apices. The bud-shaped process, which has almost freed itself from the rest, opposite the bulbous end of the notochord, is the true apex of the first visceral bar, or trabecula ; in the next stage it is far more distinct and instructive. Turning again to the vertical section (fig. 3), we see that the commissure of the trabe- culae, or inter-nasal plate, has now become a high wall of cartilage, separating not only the nasal sacs, but also the eyeballs. That part of the septum which now looks, not only downwards, but is also turned somewhat backward (fig. 3, eth.), will ultimately lie in the upper part of the nasal and frontal regions. A new thing has appeared, namely, an 