Text Appearing Before Image: GASTRULATION IN VARIOUS CHORDATA 409 in a manner similar to the pre-chordal and notochordal cells; rather, upon reaching the edge of the blastopore, it involutes over the lip, then insinuates itself between the inside entoderm and the external surface layer of cells, and, in this position, passes inward and forward between the entoderm and the external layer of cells (figs. 19IH; 198A). Text Appearing After Image: YOLK PLUG Fig. 191. Migration of the presumptive organ-forming areas of the blastula during gastrulation in the amphibia (with reference particularly to the frog). (See fig. 192.) (A) Late blastula, sagittal section through midplane of future embryo. (B-F) Observe processes of epiboly and emboly. In epiboly, the black (neural) and white (epidermal) areas become extended and gradually envelop (fig. 192A) the inward moving notochord, entoderm, and mesoderm. The processes concerned with emboly bring about the inward migration of the latter presumptive areas. (G) Late gastrular condition, with neural area and upper portion of the epidermal area removed to show relationships of the middle germ layer of chordamesoderm. (H) Horizontal section of middle gastrular condition, showing involution of mesoderm between entoderm and ectoderm. (I) Late gastrula, horizontal section, showing yolk plug, mesoderm, and final engulfment of blastocoelic space by entoderm.
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== {{int:filedesc}} == {{information |description={{en|1=<br> '''Title''': Comparative embryology of the vertebrates; with 2057 drawings and photos. grouped as 380 illus<br> '''Identifier''': comparativeembry00nels ([https://commons.wikimedia.org/w/ind...