Week 4

Major changes are seen in the development of the embryo during the fourth week. At the beginning of the week, the embryo is almost straight and will show 4-12 somites that produce surface elevations. The neural tube forms opposite the somites but it is widely open at the rostral and caudal neuropores. Typically by 24 days, the first two pharyngeal arches are visible. The first arch (mandibular arch) and second arch (hyoid arch) are distinct. The major part of the first arch gives rise to the mandible (lower jaw), and a rostral extension of the arch, the maxillary prominence, contributes to the maxilla (upper jaw). The embryo is now slightly curved due to the head and tail folds. The heart produces a large ventral prominence and begins to pump blood. 

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Three pairs of pharyngeal arches are visible by 26 days and the rostral neuropore is closed. The forebrain produces a prominent elevation of the head, and folding of the embryo has given the embryo a C-shaped curvature. Upper limb buds are recognizable by day 26-27 as small swellings on the ventrolateral body walls. The otic pits, the primordia of the internal ears, are also visible. Ectodermal thickenings (lens placodes) indicating the future lenses of the eyes are visible on the sides of the head. The fourth pair of pharyngeal arches and the lower limb buds are visible by the end of the fourth week and a long tail-like caudal eminence is a characteristic feature. Rudiments of many organ systems, especially the cardiovascular system are established. By the end of the fourth week the caudal neuropore is
usually closed.

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