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Show 5/8 MR. J. B. SUTTON ON THE DEVELOPMENT [June 2, extending from the margin of the foramen magnum to the summit of the dorsum sellae-is composed of the same elements as the vertebral column, but differs from it in that it has not passed through any stage of vertebration1, the notochordal portion with the parachordals representing the centrum, with the laminae which meet dorsally in man and mammals, over the developing brain in the occipital segment only. Anteriorly the representatives of the laminae take on a very different disposition. During development the skull, whose long axis was originally a direct continuation of, and in the same plane as, the vertebral column, becomes at an early period bent, or, as it is usually described, flexed downwards. One of the important results of this flexion is the dissociation of the anterior portion of the lateral Fig. 1. A diagram to represent the disposition of parts in the base of the primitive skull. N.C, Notochord; Pa, parachordals; P.C, periotic capsules; T, trabeculae; CT, ethmo-vomerine region. neural walls from the parts immediately adjacent. Eventually these dismembered portions of the neural walls coalesce around the down-bent brain and are recognized as the trabeculae cranii. This admirable explanation was first promulgated by Goette (Entwicklung-geschichte der Unke, page 629) ; and this view has certainly much more to recommend it than the notion that the trabeculae are to be regarded as a pair of branchial arches. 1 Vide Huxley, " The Cranio-facial Apparatus of Petromyzon," Journal Anatomy and Physiology, TOI. X. p. 418. |
