Figure 1-31 The primitive intestinal tube is shown at three stages of its development (A-C) during the third, fourth, and eighth weeks of gestation. Before the formation of the head fold, during the third week the yolk sac is an ovoid cavity. Its roof is the endoderm, which is the underlayer of the embryonic disk. With the formation of the head fold during the fourth week, a portion of the yolk sac becomes included within the embryo. This results in an endodermal tube dorsal to the pericardial cavity and the septum transversum; it adopts a medial position. The tissues of the cranial foregut form the buccopharyngeal membrane, which separates the future digestive tube from the primitive mouth, the stomodeum. Laterally, the foregut is bounded by the bronchial mesoderm. Rapid growth of the brain with transverse and sagittal folding during the fifth week results in the apparent flexion of the embryo. Simultaneous constriction at the junction between the embryo and the yolk sac separates the primitive midgut from the yolk sac remnant. The amniotic cavity expands and obliterates the extraembryonic coelom. (1 = embryo, 2 = yolk sac cavity, 3 = amniotic cavity, 4 = extraembryonic coelom, 5 = cytotrophoblast and extraembryonic mesenchyme, 6 = somatopleure, 7 = splanchnopleure, 8 = septum transversum, 9 = cardiac tube.)
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Figure 1-33 Sagittal section diagram through human embryos of different stages. The digestive tract and its accessory glands undergo rapid development between the twenty-fifth and the thirty-fifth days. (1 = head, 2 = pharynx, 3 = tracheal bud, 4 = esophagus, 5 = stomach, 6 = pancreas, 7 = liver, 8 = heart, a = foregut, b = midgut, c = hindgut.) The septum transversum and the buccopharyngeal membrane are indicated by short and curved arrows, respectively.
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