The human embryo at about 6 weeks, surrounded by the fluid-filled amniotic sac, with the remains of the yolk sac still attached (above head). The umbilicus (carrying 2 arteries & 1 vein to the placenta) connects the embryo with the maternal circulation, supplying oxygen & nourishment & removing waste materials. At this time, most of the organ systems are fully formed: the retina of the eye is visible (dark spot), the arms & legs have budded; the heart has formed & has been beating since the 3rd week. The brain is in 3 parts, the fore-, mid,- & hindbrain, with a large void at the base; this will be partially filled by further growth (the remnant is the 4th ventricle).