The most characteristic event occurring during the third week of gestation is gastrulation, the process that establishes all three germ layers (ectoderm, mesoderm, and endoderm) in the embryo. Gastrulation begins with formation of the primitive streak on the surface of the epiblast (Figs. 4.1-4.3 A). Initially, the streak is vaguely defined (Fig. 4.1), but in a 15- to 16-day embryo, it is clearly visible as a narrow groove with slightly bulging regions on either side (Fig. 4.2). The cephalic end of the streak, the primitive node, consists of a slightly elevated area surrounding the small primitive pit (Fig. 4.3). Cells of the epiblast migrate toward the primitive streak (Fig. 4.3). Upon arrival in the region of the streak, they become flask-shaped, detach from the epiblast, and slip beneath it (Fig. 4.3, B-D). This inward movement is known as invagination. Once the cells have invaginated, some displace the hypoblast, creating the embryonic endoderm, and others come to lie between the epiblast and newly created endoderm to form mesoderm. Cells remaining in the epiblast then form ectoderm. Thus, the epiblast, through the process of gastrulation, is the source of all of the germ layers (Fig. 4.3B), and cells in these layers will give rise to all of the tissues and organs in the embryo.
As more and more cells move between the epiblast and hypoblast layers, they begin to spread laterally and cephalad (Fig. 4.3). Gradually,
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