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Titre : Philosophical transactions of the Royal society of London. B

Auteur : Royal society (GB)

Éditeur : printed by Harrison and sons (London)

Éditeur : The Royal society (London)

Date d'édition : 1908

Type : texte

Type : publication en série imprimée

Langue : Anglais

Format : application/pdf

Description : 1908 (VOL199).

Droits : domaine public

Identifiant : ark:/12148/bpt6k56064p

Source : Bibliothèque nationale de France

Relation : http://catalogue.bnf.fr/ark:/12148/cb34422226r

Relation : http://gallica.bnf.fr/ark:/12148/cb34422226r/date

Provenance : Bibliothèque nationale de France

Date de mise en ligne : 15/10/2007

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124 PROFESSORS J. T. WILSON --AND J. P. IIILL. Posteriorly the ganglionie plate under considération shows nô definite

Posteriorly the ganglionic plate under considération shows nô definite limitation, and appears simply to merge in the region of the. first somite,1 If regard were had only to surface-photographs such as those reproduced in figs..75 and 7G,.it would nàturally be supposed that that which we here describe as a ganglionic plate in reality merely "rj&pfesents the forward extension of the paraxial zone of mesoderm. Fig. 75. is more especially liable to such au Interprétation, inasniuch as the surfaceview can hardly be said to yield any suggestion of the .piesençe of such a subectodermal ganglionic plate as that described. Here, on each si.ile of the medullary plate., we find an oblong area boUnded behind by the transverse cleavage-line defiriing the first distinct somite laterally bya tolerably definite boundary which is. coïncident with thé inner limit of the pariétal crelomic cavity medially -by the edge of the .medullary- plate and anteriorly by a well-marked opaque band, parallel with the' pediclèVf thé acustico-facial ganglion, and skirting the poste rior part of the periphery of the circulai' auditory plate. Thé broad elongated area so defined is in tolerably,, obvious séries with the great Trigeminus- plate in front and. also with the acusticofacial ganglion and its pèdjele. Tirât 'it is not to be interpreted as the forward ?prolongation of thev.paraxial zone- -6fr:'mêso(|gr^i,f^eyjdeneed iiy the fiict that the true anterior prolongation of the somitic seiies ???itâtîlï ^^X»-isible, in both of the figures (75 and 76), in the shape of a pointed proïoiigatidii'of. somitic tissue, occupying- only the medial posterior quarter of the oblohg area ? iîi question, and tapering off hère into a pointed anterior termination. ? «t^ Such a négative, of course, in no way involves or supplies a démonstration of thé ganglionic character of the cellular material of the area in-quèstion. The .existence of a ganglionic lamina in the sub-ectpdermal plane of the elongated inteiwal between^fce auditory region and tlîat of the first somite, is rather difBcult to realise, unless recourse be had.to study of the sections. Thé conception ofthe existence of such a plate is slightly less ditticult in the case of the right side of fig. 76. On both sides of the figure, but more especially on the left, the thin and .imperfect nature of the lamina, except in its anterior and latéral marginal portions, is revealed in thé relative liglithess jtnd transparenéy of the remainder of thé area occujûed by the plate. It is owing to this that the ganglionic rudiment seems on surface-view- of tins sjjecimen to possess the form of an opaqua strand running ] ouUyards and then backwards, behind the auditory region, to fade away in thé région of the first somite. It has already been pointed out in ^connection with the èarlv neurular phase that the aj'cuate band of opacity visible in the corresponding ?situation "in fig. 74: is to be interpreted as thé thickened and convex edge of a ganglionic plate or lamina.- This thiekened and convex margin is already less sharp and d( imite m fig 76 than it, was in fig. 74. But in fig.' 75, from Specimen^M, it is onlv the «uiterior margin of the plate which is clearly defined as such. Thé liteiaî mai gin, which here runs backwards at right angles with t^e anterior margin, is now practically coincident with, and on surface-view indistinguishable from, the