The head enlarges considerably and exhibits bulges produced by the cerebral vesicles. As the head raises from the ventral chest wall, the neck becomes better defined. A redundant layer of ectoderm called the cervical fold is sometimes visible in the cervicomandibular area.
The optic cup begins to migrate from the side of the head to the face. Upper and lower eyelid folds appear in the ectoderm above and below the cup, respectively.
The branchial arches and grooves disappear as the opercular fold covers over the more caudal arches. The first groove is surrounded by hillocks from which the auricle develops. The proximal part of the maxillary and mandibular processes blend with each other forming the cheek. The primitive rima oris is located between the distal part of the two processes. The surface features of the face are described in Section III, A.
The arm, forearm and hand become evident in the upper limb bud. The thigh, leg and foot appear in the lower limb bud. Five ridges separated by grooves form at the periphery of the primitive hand and foot plates and represent the digits. The limb buds rotate in such a way that what was formerly the ventral surface now faces medially. The ventral surface or palm of the hand is adjacent to the chest wall. The ventral surface or sole of the foot lies adjacent to the umbilical cord. Thenar and hypothenar areas of the palm are represented by elevations at the base of the thumb and little fingers, respectively.
The heart and liver form prominences in the ventral body wall.
A small bud on the lateral chest wall called the mammary gland primordium is the only remnant of the mammary ridge.
Although the umbilical cord remains very large, its size relative to the embryo is reduced.
The surface features of the external genitalia (perineal region) are described in Section IV, E.
All that remains of the tail bud is a short, blunt projection in the midline between the buttocks.
Source: Atlas of Human Embryos.