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Show 242 ON 1,HE STRUCrruRE OF THE SKUJ,J,. ossi· ry . At, a lI't tle dI'S t anc-e from the occipital foramen, there arises a very small scinilunar bony plate, the concave edge or excava-tion of which js directed forwards; thereupon, the bony subst.ance shoots from this edge further and further forwards, until at length the bony plate has the forn1 of tho ace of hear~s. 1~ts b '- borders the fontanelle in the base of the skull, wlnch IeR ase . I h'l . under the anterior half of the third cerebral ves1c e, w I e 1ts point is contignous to th~ occi~ital foramen; for tl.1e ~ost part 't · ery thin and only Its axis (and next to thiS Its whole I Is v , · l Tl posterior margin) is distinguished by a greate~· tln~ n1ess. . 1e cephalic part of the notochord can be .recogmsoa Ill the aXIS of this bony plate up to the following penod. It passes ~r~1n tho posterior to the anterior end of the bony plate, whore It IS lost, and is so invested by the os. eous substance of the plate: that a smaller portion of the latter lies on the ?pper side o!' the notochord, a Iaro·er portion beneath it. On this account 1t forms, on the npp81~ side of the plate, a longitudinal rid~e, which subsequently becon1es imperceptible by tho aggregatiOn of matter at the sides. On one occasion, however, I saw, in an embryo which had almost reached its full tern1, a similarly formed and sized bony cone, which, through almost its entire length, appeared merely to lie on the body of the basi-occipital, since it had only coalesced with it below. The nucleus and sheath of the cephalic part of the notochord become gradually broken up and the last trace of them eradicated, as the ossification of the basi-occipital proceeds, like the nucleus and sheath of the rest of the notochord wherever a vertebral body is developed.* . . The articular condyle is not yet formed. rrhe ex-ocCipltals ossify through their whole length and breadth. . The body of the basi-sphenoid is formed between the abovementioned posterior fontanelle of the basis eranii and the pituitary space, therefore far from the cephalic part of. the notochord. It ossifies by two lateral centres, each of whwh forms * In tho Stickleback i.t has appeared to me tl1at tho wall of tho anterior conicn.l termination of the notochord in 1ho basis cmnii becomes ossified, or, nt nny rate, invested by o.n inseparable s11onth of bony mntter, jnst in the same way rts tho "uro. tyle '' is developed in tllG tAil. '!'HE SH.:UJJLS OF REP'l'JLIA AND AVES. 243 a ring round the carotid eanal. * The alisphenoids ossify in their whole length and breadth; the orbito- phenoid only slightly, and the presphenoid not at all. The premaxillary bone arises as an azygos triangular cartilage between tho cornua of the anterim~ ethmo-v01nerine plate. It ossifies from a single centre. The auditory capsule, or the future petrosal [ = periotic J bone, may, even at the end of this period, be readily separated fron1 the other part of the cranial wall, and still consist , for the most part, of cartilage. On the other hand, the triangular form, which it had before, is not inconsiderably altered, since it greatly elongates forwards, and thus, as it were, thrusts its anterior angle further anc1 further forwards, and becomes more unequalsided. At the lower edge, or the longer side of it, ·about opposite to the upper angle, at the beginning of this (third) period, or indeed smnewhat earlier, a diverticulum of the auditory capsule begins to be for1nod (the rudimentary cochlea), and develops into a moderately long, blunt, and hollow appendage, the end of which is directed downwards, inwards and backwards, and also consists of cartilage. · Close above, and somewhat behind this appendage, however, there appears, at about the same time, a small rounded depression, in which the upper end of the auditory ossicle eventually rests ; and, so mew hat later, an opening appears in this depression which corresponds with the fenestra rotunda of man. Very much later, nfunely, towards the end of this period, the auditory capsule begins to ossify. Ossification commences in a thin and moderately long, hook-like process, which is sent forwards and inwards from the lower hollow diverticulum of the cartilage, and unites with the basisphenoid. From this point it passes upwards and backwards, and, for the present, extends so far that, at the end of this period, besides that process, the diverticulum in question, and about the anterior third of the auditory capsule itself, are ossified. t Later than at the point indicated, an ossific centre appears at the posterior edge of the auditory capsule, where it abuts against the supra- and ex-occipitals, but extends from hence by no means so far forward as to meet that fro1n the * These are the "basi-temporals" of Mr. Parker. t This is the pro-otic ossification. n2. |