The human embryo at about 7 weeks, measuring about 3cm. Most of the primitive organ systems & external appendages are formed: the arms & legs have budded (the toes are visible); the retina of the eye & nose may be seen. The large dark mass in the body cavity 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 void at the base of the head has yet to be filled by further growth. The umbilical cord (top right) connects with the placenta, an organ rich in blood vessels which interfaces with the maternal circulation, exchanging nourishment & waste products.