. The chordates. Chordata. Fig. 211. Gastrulation in the frog. (A, B, C) The whole embryo at successively later stages; viewed toward the vegetal pole. (A', B', C'j Bepresent, in a somewhat diagrammatic way, sections of corresponding stages cut in the plane including the polar axis and bisecting the gastrular invagination, (I); this plane corresponds to the median plane of the adult. During the latter part of the period of gastrulation, as the result of shifting of the heavy yolk (compare B' and C), the embryo rotates so that the axis passing through the blastopore (BP) becomes horizontal (see
. The chordates. Chordata. Fig. 211. Gastrulation in the frog. (A, B, C) The whole embryo at successively later stages; viewed toward the vegetal pole. (A', B', C'j Bepresent, in a somewhat diagrammatic way, sections of corresponding stages cut in the plane including the polar axis and bisecting the gastrular invagination, (I); this plane corresponds to the median plane of the adult. During the latter part of the period of gastrulation, as the result of shifting of the heavy yolk (compare B' and C), the embryo rotates so that the axis passing through the blastopore (BP) becomes horizontal (see Fig. 215A). (a) Archenteron; (an) animal pole; (b) blastocoele; (bp) blastopore; (ec) ectoderm, (en) endoderm; (i) invagination; (np) neural plate; (veg) vegetal pole; (y) yolk; (yp) yolk plug. (Courtesy, Neal and Band: "Chordate Anatomy," Philadelphia, The Blakiston Company.). 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 Rand, Herbert W. (Herbert Wilbur), 1872-1960. Philadelphia : Blakiston
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