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P.806 3rd paragraph: "Formation of the heart tube: The mesodermal tissues that give rise to the heart first become evident when the embryo is undergoing the process known as gastrulation. In the human, this occurs during the third week of development, while for the mouse, at a comparable stage of development, around seven days will have elapsed from fertilisation, and the embryo will be in the presomitic stage. The embryonic plate in humans, initially possessing two layers, is ovoid, and is formed at the union between the yolk sac and the amniotic cavity. In the midline of the long axis of the oval disc is found the primitive streak, with the node at its cranial end. Through this streak, cells migrate from the upper layer by the process called gastrulation to form the three germ layers of the embryo proper: the ectoderm, the endoderm, and the mesoderm. The mesoderm insinuates between the ectodermal and endodermal layers, which themselves are continuous with the amnion and yolk sac, respectively. Having insinuated, the mesoderm spreads laterally and cranially within the embryonic disc, ultimately giving rise to a variety of structures, such as the somites, which will produce the axial structures, and the lateral plate mesoderm, which will form the parietal body wall. The cells that are destined to form the heart are also derived from this mesodermal layer. They form a crescent virtually at the cranial border of the disc (fig 1). As this heart forming region achieves its crescentic shape, the central region of the ectoderm transforms into the neural plate. This folds to become the neural tube, with the developing brain at its cranial end. In the human, the developing heart is initially cranial within the disc relative to the neural folds." |