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The specimens measure approximately 4 to 8 mm in length and have up to 42 to 44 pairs of somites.

The configuration of the embryo changes greatly, bending ventrally into a C shape.

The lateral, head and tail body folds move the attachment of the amnion to the midventral area in contact with the umbilical cord. In this area the coelom communicates with the extraembryonic coelom through a narrow channel between the attachment of the amnion and the yolk sac.

Bulges of the ectoderm form prominences in the head region that indicate the position of several structures.

The rostral and caudal neuropores have closed and are no longer evident.

Up to four branchial arches and grooves become distinct in the future lower face and neck region. The stomodeum or primitive mouth is bound by the maxillary and mandibular processes of the first arch. The fourth arch is in the floor of a depressed area called the cervical sinus.

The heart is prominent and located ventrally between the branchial arches and the yolk sac. The atrial and ventricular chambers are large and can be distinguished through the ectoderm.

The mesonephros, umbilical veins and spinal cord form bulges in the ectoderm.

The upper and lower limb buds appear as blunt projections on the lateral surface. A blunt, fingerlike tail bud becomes prominent in the midline in the most caudal part of the embryo.

The cloacal membrane is located in the midline on the ventral side between the lower limb buds.

Source: Atlas of Human Embryos.