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Show ON THE S'I'R' UCTUHB OF 'l'JJE H K UJ~T~. 140 l . l d r Tho fill dI. to r. y 11 rv is d. vclop c In t 1 being· only secon a Y· I . ·d c r bral v I 1 , o that the d. t to the tnt . h blastoder.m a f JHCen e-capsn1 e s d o n ot correspond with the t roe three pairs o sens . Primary cerebral veswles. 1 been croina on in the proper . I hanaes 1ave o b . While t wse c b b, tho rudim nts of th face lw.v . 1 -t. of the em I vo, . A l crania poi 1011 l · . p arance nn d. I , a v ry. sino·n lar gm e. s t w made t 1ell' afp th<- dorsal I tUni. n, I· ll th<: _, hrad hav oTown up-homologues 0 e h 1. 'ty 0 . 1 the cep a IC ·avi ' ' plate's' which orrespond wards to In·c ose I Janu.n ' m of the t 'llnk hav oTown downward , I ' b with the· Vtl ctehra 0 ·terr.o r wa ll s o f the Luecal ' I haryng nJ , au.d to co.n st1tu e.· e PT he e . . 1 1 t s how " r clo 1Jot romam VI c Ia p a ' ' cerviCal reowus.. . d tho of th trunk, lmt groove. entir and nndivi.ded, a . I to th uxi ., of the trunk, · h dr · cted tran v I Y . appear In t em, 1 • t ally b . 0 111 1 .. , s deepernng, onYertecl Into V n 11 ' · and, t le gr~o ,~ -whi ·hop n into the pharyng al ca_vJt~, slits-the Vttsceral cleJ 'ts . l . 7 rfhe fir t ht 1~ dina vzsce?·a ar; CtWS. a·n d bdo un· md mcemdi:areteslp yo nb ol bow an<.1 in fro· nt of th auditory sa· r, Situate I t tl ' :fir t ancl , on d VI· 'ei'("(, 1 (c u·ch ' -the anten or and separa f s tl lC former b m.n cl t J'II1in cl by· tho clg s of a (Y boundar· y o f tll e .n t o·urnont bw 1 u . ·h WI' ll v utnally lle ·on1e tho depres wn ? ~ . ~ r:: b C) A. third fourth ancl fifth vi. cera] buccal cavity (Fig. 07 ' · . . 1 I' b hind th fir t and d l l in UCC ' 'lV OH - arch da r(eF v opec F' . 9 F) . hut a ... th y ar of no great · 57 l) and 1 l 0 • -' ' c • b .m. ecoomne nt inIg .r efe'r en'c e to t hbe 1l uman l\,... ul] ' our attention n1ay e confined to the latt r. f f , that fr m th mom nt at It i particularly worthy o . n~ we ( ' 1 . t f the fir. t which I. t . d' 'bl a di tinct part, t l lOO o 1s Iscerm e a . .. · b low anti in . t tl rtH.lun ntarv ld.Uinin ' visceral arch pa. se m 0 · 1 . • 1 'le th <' l'Oot of the front of~ the for part of th anchtm~yl.· twi,. 'tv l: vVe hall find . I 1 bell and b huH. t ut tW. . second IS atta(' ICt e ow . < • ·m·he retain the ·amc posJ-that the parts developed witlnn thl( , tho i . which involves . . th d lt state . o that any lYP . . tion In e a u ' ' f' 1 of th -' 0 . . f' t ·v hancre o p ac parts In the uppo ltiOn o an ex n L n tl . f con ideration. the course of deveI opinont I· ' ~·p so J~ 'acto nn wor 1y l ' ected with Both the first and s con l VI· era1 ai· chcs are conn . b t the that part of the cram.u m wlu.; h rI ~s Le 111 ·1 1 d ft htoh efl ebxounrde , exhui bits, inflected portion of the f'raninm ln front o THE DEVEI,OPMENT OF 'l'IIE FOWL'B SKULL. 141 on each side, running from the root of the :fir t visceral arch beneath tho eye to the nasal sac, a ridge or elevation, which is called the maxillary process, and mjght be regaTded as a vi ceral arch of the anterior division of the skull, from the ba ·e of which it is developed (Fig. 32, G, and l, Fig. 57, F). Lastly, the middle part of the floor of the anterior cerebral vesicle, between the , nasal sacs, thickens and gives rise to a broad, flat median process, with an expanded extremity, the terminal contour of which is excavated and slightly produced at the angles-the fronto-nasal pr·ocess (Fig. 57, F, k). At first, the cranium and all its arches are membranous, or composed of mere indifferent tissue, with the exception of the axial notochord; but, very early, chondrification commences. The indifferent tissue snrrounding the notochord (the "investing mass" of Rathke) (Fig. 57, 0, D,j), is converted into cartilage, and the same histological change takes place in the walls of the auditory capsules, and around tho foramen magnum; the cartilage stops in the middle line, behind the pituitary body, but sends two processes, one on each side of that body, into the floor of the anterior division of the skull (Fig. 57, F', tr). These processes, the trabeculm cranii, of Rathke, unite in front, and the cartilage formed by their union ends in the fronto-uasal process. The roof of the skull, and the greater part of its side~ walls, except in tho region of the foramen magnum, are, at first, entirely membranous. Chondrification next takes place in the visceral arches; a rod of that substance, which coalesces with its fellow in the middle line, being formed in the axis of the several arches on each side. Purposing to return to the visceral arches by and by, I shall now trace out the modifications which are undergone by the chondro-membranous brain-case. In the occipital region, and about the auditory capsules, which early attain a very large proportional size, the cartilage extends for some distance upon the infero-lateral parietes of the skull; on the floor of the posterior division of the skull it thickens notably, and forms a sort of model of the future basi-occipital and basi-sphenoidal regions, the interspace between the trabeculre becoming rapidly obliterated and converted into the floor of the pituitary fossa. |