. Amphioxus and the ancestry of the vertebrates. Amphioxus; Sea squirts; Hemichordata. INTERNAL ANATOMY. 91 the ventral tube, so as to form a vesicular dilatation cov- ered over by a thin membrane. The region of the nerve- tube, over which this dorsal dilatation extends, has been compared by Hatschek, who discovered it, to the medulla oblongata of the craniate Vertebrates, which is similarly roofed in only by membrane. In the fully grown condi- tion, however, it seems to be largely obliterated by the. Fig. 46.—Transverse section tlitongh the spinal cord between ttie second and thiird sensory r
. Amphioxus and the ancestry of the vertebrates. Amphioxus; Sea squirts; Hemichordata. INTERNAL ANATOMY. 91 the ventral tube, so as to form a vesicular dilatation cov- ered over by a thin membrane. The region of the nerve- tube, over which this dorsal dilatation extends, has been compared by Hatschek, who discovered it, to the medulla oblongata of the craniate Vertebrates, which is similarly roofed in only by membrane. In the fully grown condi- tion, however, it seems to be largely obliterated by the. Fig. 46.—Transverse section tlitongh the spinal cord between ttie second and thiird sensory roots. (Ai"ter ROHDE.) Dorsal aggregation of ganglion-ceils {extending between the second and fifth pairs of sensory nerves ; a somewhat similar group of ganglion-cells occurs on ventral side of nerve-cord below the central canal between the fourth and sixth sensory nerves.) Dorsal root, Supporting fibres, ex. central canal; in this case equally wide throughout its entire height, and so all along the spinal cord. sh. Sheath of nerve-cord. development of a mass of large ganglion-cells which ex~ tend backwards as far as the fifth pair of sensory nerves (Fig. 46). All there is of a brain in Amphioxus is shown in Fig. 45. The cerebral vesicle is a plain cavity without any true subdivision into ventricles.^ In the development of. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Willey, Arthur, 1867-. New York, London, Macmillan
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