. Amphioxus and the ancestry of the vertebrates [microform]. Vertebrates; Chordata; Fishes; Ascidiacea; Vertébrés; Cordés; Poissons; Ascidiacés. 198 THE Formation of Medullary Tube and Notochord. Here, as in Amphioxus, the dorsal wall of the embryo flattens, while the ventral remains convex, and the ecto- dermic cells on the tlorsal side become marked off from the rest by their larger size and columnar shape. The medullary plate extends nearly to the front end of the embryo, while posteriorly its cells form a ring round the blastopore. In the formation of the medullary tube, however
. Amphioxus and the ancestry of the vertebrates [microform]. Vertebrates; Chordata; Fishes; Ascidiacea; Vertébrés; Cordés; Poissons; Ascidiacés. 198 THE Formation of Medullary Tube and Notochord. Here, as in Amphioxus, the dorsal wall of the embryo flattens, while the ventral remains convex, and the ecto- dermic cells on the tlorsal side become marked off from the rest by their larger size and columnar shape. The medullary plate extends nearly to the front end of the embryo, while posteriorly its cells form a ring round the blastopore. In the formation of the medullary tube, however, there is an important difference, and the Ascidian embryo con- forms in this point more to the mode of development. Fig. 98. — Transverse sections through embryo of Clavelina Rissoana, to show mode of formation of medullary tube and mesoderm. (After Davidokk.) A. Through anterior region of embryo, with medullary groove still open. n, Throu<;h posterior region, with closed medullary tube. c/i. Rudiment of notochord. ec. Ectoderm, en, Endoderm. mes. Mesoderm. Medullary groove, tnj. 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, 1867-1942. New York; London : Mcmillan
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectfishes, bookyear1894
