Stage 18 CHAT Stage 18 Ossification of the Skeleton Begins Head and Neck Nerve plexuses begin to develop in the region of the scalp. Eyes are pigmented and eyelids begin to develop and may fold. Thorax Within the heart, the trunk of the pulmonary artery separates from the trunk of the aorta. Nipples appear on the chest.
Body appears more like a cube.
Abdomen Kidneys begin to produce urine for the first time. Pelvis Genital tubercle, urogenital membrane and anal membrane appear. The critical period of arm development ends, and the arms are at their proper location, roughly proportional to the embryo. However, the hand plates are not finished, but develop further in the next two days. The wrist is clearly visible and the hands already have ridges or notches indicating the future separation of the fingers and the thumbs.