The human embryo at about 7 weeks, protected in the fluid-filled amniotic sac. Most of the primitive organ systems & external appendages are formed: arms & legs have budded (fingers are visible); the retina of the eye & nose may be seen. The large mass in the body cavity (to the left of the hand) is the enlarged liver, which manufactures red blood cells until the bone marrow (as yet undeveloped) takes over. The brain is undergoing radical divisions. The umbilical cord (centre) connects with the placenta, an organ rich in blood vessels, which interfaces with the maternal circulation, exchanging nourishment & waste products.