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Show - OF 'rHE VEHTEBHA1'11~ SKULl), 292 ON THE STRUCTURE 1 1. c part of tho notochord like a 1nass whieh surrounds tho cep la 1 t f 't which lies between · 1 · d · that par o 1 sheath; the basl-sp JenOI ' Jnb cl ) d the anterior end of the · d s (the ira ecu IE an the paire procdesshe tl 'd (more particularly its body, or pars otochord . an t e e 1ID01 n d. 'z · ) · the anterior coalesced part of those two pro- p er'YJen wu ar2s ' lll h h d . cesrs es. rT he b 0 d Y 0 f the l)resphenoid, on the ot er an , IS . 1 £ . d below the processes in questiOn, rarely between t lem. 01 ~,e G.) 'rhe parts of the skull just mentioned, however, do no~ 'f (. ll TT t brata with an osseous skeleton, but one, OI ossi y l n a r er e . . . d 1 l f. th sometiJnes remain cartilaginous, an t l en grow severa ' o em t b l , . f: • l th the others so that they seem o e pus lO(L relatively ar ess an ' · h ld asi.d e and suppr.e sse d by the neighbouring bones.. Tluds fo l s good espem.a ll Y of the basi-occipital of the Batra. ch~a, .a n o t le b . l 'd of these animals and of osseous Fishes. ~~~ill ' f "(7.) The basi-occipital (or: at le~st, the ~u?stance. out. o which it will become developed) constitutes, onginally, hke the body 0 f avert eb r.,a , a sheath round a part of the notoehord, and t h e ex-ocm.p 1' ta ls appear whilst they chondrify, as outgrowths £ ' from t l 1e b asr·- ocm·p r' tac l p:1rt '· J·ust as the arches of a vertebr•a , when t hI.S I·S no1..m a lly developed ' appear as outgrowths from Its a l reacl y ch on dl.1'fied body · ~-,or the . rest,. how. ev.e r, the normfa l d eve l opment of the occipital bone IS qtute . sim. ilar to that · o ba ver t eb ra, and I·t therefore mavJ with perfe•c t • JU• StiCe •b e held to • e a cep1 l al l.C ver· tebra. * The sq1tama occ~'rY Jd~s, wh wh occurs md b t not in all Vertebrata, and which is not always place many, u · · 1 t b et wee n , but sometimes lies in front of the. ex-oc. Ci.p ita s, preson. s 110 difficulty in the way of this interpretatiOn; It IS an aecoss~I Y structure, a so-called intercalary bone, the prese~ce of whiCh depends upon the excessive development of the br~In. "(8.) rfhe two rings, on the other hand, w}uch are fo:In?cl by the two sphenoids, with the parietals and frontal~ as their mtercalary bones, are no longer constructed upon q~Ite the sa:no t e as the vertebra. That. thA alisphenoids and orbito-sphonOids, :fen they are already chondrified, do not a~pear ~ take the form of outgrowths of their centres, but are unrted ~·Ith them by membrane, need not, perhaps, be taken very much Into account, · •t 1 f n,ny Verfe- * The ·F oramt·n a con d YZ o~; dea • which occur in the ex-ocmp1 a. s o m bt at a, rcm1· n d one of the holes of the vertebral arches of the Sha.rks. THE THEORY OF THE VERTEBRATE SKULL. 293 since, in the La1npreys, the arches of the vertebrre arise independently within the lamellre, which the investing mass of the notochord has sent out to embrace the central parts of the nervous system. Still less weight can be attached to the circumstance that not unusually, even when both sphenoidal centra are present, only one pair of the corresponding alre appears ; while, in other cases, two pair of alm and only one central part are present, since the caudal vertebrre of Mammalia usually exhibit no traces of arches, and the Lampreys have such arches without centra. On the other hand, the circumstance is important that the basi-sphenoid, although it arir;es within the investing mass of the notochord, is not developed around this (as, so far as our present observations go, even the most posterior caudal vertebrre are), but in front of it, in a process of the investing mass; and that the body of the presphenoid is no longer developed, even in a part of this mass (except in a few Mammalia), but arises quite independently of it. Hence, the two sphenoids no longer agree perfectly with vertebrre in their development-the anterior diverging more widely from the vertebral type than the posterior. "(9.) Yet the two sphenoids, like the proper vertebrre, still embrace segments of the nervous tube (such as is formed by the brain and spinal marrow, at any rate in the early stages of development), and they constitute, as the vertebrre at first normally do, open rings, or rather segments of rings, round that tube. The ethmoid, however, at no time surrounds a segment of the nervous tube in question; but, in a few animals only, imperfectly includes, by its hinder part, two anterior prolongations of that tube, whence the olfactory nerves arise. Its mode of development, and its ultimate form likewise, are of such a character that it no longer offers any special resemblance to a typicallyformed vertebra. Nevertheless, considering that it arises frmn a part of the prolonged investing mass of the notochord-viz., from the anterior, early-coalescing parts of the two trabeoultEand that its body (the pars perpendio~daris) presents even a certain resemblanee to the last caudal vertebrre of many Birds and osseous Fishes, it may well be considered to be a modified vertebra. We may look at it, in short, as the representative of only the |
