. Amphioxus and the ancestry of the vertebrates. Amphioxus; Sea squirts; Hemichordata. Fig. 98.—Transverse sections through embryo of Clavelina Rissoana, to show mode of formation of medullary tube and mesoderm. (After Davidoff.) A. Through anterior region of embryo, with medullary groove still open. B. Through posterior region, with closed medullary tube, ch. Rudiment of notochord. ec. Ectoderm, en, Endoderm. mes. Mesoderm. Medullary groove. Medullary tube. which is typical of the higher Vertebrates than does Amphioxus. In the latter the medullary plate sinks bodily below the level
. Amphioxus and the ancestry of the vertebrates. Amphioxus; Sea squirts; Hemichordata. Fig. 98.—Transverse sections through embryo of Clavelina Rissoana, to show mode of formation of medullary tube and mesoderm. (After Davidoff.) A. Through anterior region of embryo, with medullary groove still open. B. Through posterior region, with closed medullary tube, ch. Rudiment of notochord. ec. Ectoderm, en, Endoderm. mes. Mesoderm. Medullary groove. Medullary tube. which is typical of the higher Vertebrates than does Amphioxus. In the latter the medullary plate sinks bodily below the level of the surrounding ectoderm, which then grows over it. Subsequently while underneath the ectoderm the medullary plate assumes the form of a half-canal open towards the ectoderm, and eventually its margins come together and so form a complete tube. In the Ascidian embryo the overgrowth of the surround- ing ectoderm and the folding up of the margins of the. 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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