. Comparative embryology of the vertebrates; with 2057 drawings and photos. grouped as 380 illus. Vertebrates -- Embryology; Comparative embryology. EXTERNAL OPENING OF NOTOCHORDAL CANAL. Fig. 201. Transverse sections of the late turtle gastrula as indicated by lines in fig. 199C. elongated antero-posteriorly by the forces of epiboly. Meanwhile, the external opening of the notochordal canal changes in shape and together with the primitive plate moves caudally (fig. 199). As gastrulation draws to a close, the neural plate area begins to fold inward, initiating the formation of the neural tube.
. Comparative embryology of the vertebrates; with 2057 drawings and photos. grouped as 380 illus. Vertebrates -- Embryology; Comparative embryology. EXTERNAL OPENING OF NOTOCHORDAL CANAL. Fig. 201. Transverse sections of the late turtle gastrula as indicated by lines in fig. 199C. elongated antero-posteriorly by the forces of epiboly. Meanwhile, the external opening of the notochordal canal changes in shape and together with the primitive plate moves caudally (fig. 199). As gastrulation draws to a close, the neural plate area begins to fold inward, initiating the formation of the neural tube. 4. Gastrulation in the Chick a. Orientation As described in Chapter 7, a two-layered blastoderm (blastula) composed of an epiblast and a hypoblast is present, with the hypoblast more complete at the posterior end of the blastoderm than at its extreme anterior and antero- lateral margins (figs. 171 A; 202A). The epiblast over the posterior half of the blastoderm is composed of presumptive notochordal and mesodermal cells, and anteriorly in the epiblast are found the presumptive epidermal and neural areas (figs. 173A; 202A). b. Gastrulative Changes 1) Development of Primitive Streak as Viewed from the Surface of Stained Blastoderms. The formation of the primitive streak is a progressive affair. Figure 170 pictures a pre-streak blastoderm, and it is to be observed that the ectodermal layer below the epiblast is present as an irregular area in 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 Nelsen, Olin E. (Olin Everett), b. 1898. New York, Blakiston
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