To be accurate, gastrulation begins at 51/4 h; this process comprises the cell movements of involution, convergence and extension (35), which are initiated when epiboly has reached 50% (50% coverage of the yolk cell; Fig. 2C). At the onset of gastrulation, the marginal zone becomes thickened to form the germ ring, one region of which, at the site of dorsal axial involution, becomes distinctively thickened. This localized thickening, caused by convergent movement of cells to the axial (dorsal) location, is called the shield (Fig. 2D). Gastrulation movements lead initially to the formation of two layers, the inner hypoblast (mesendoderm),and the epiblast (ectoderm) (Fig. 2E). Understanding the cell movements of gastrulation is aided by appreciation of the fate maps that are available for this period (1,3).
During gastrulation, cells in the axial midline extend anteriorly to create the notochord and the most anteriorly located prechordal plate (Fig. 2F). This anterior region produces the hatching gland and the pharyngeal endoderm. More laterally, the rostral hypoblast forms the muscles of the head and, more caudally, the somites. By the end of gastrulation epiboly is complete and the tail bud has formed (Fig. 2F); in addition,the ectodermally derived neural plate is beginning to be formed on the dorsal side of the gastrula (36).
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