The human embryo at around 7 weeks, surrounded by the membranes of the chorion (outer layer), & the amnion (the inner, fluid-filled layer). At this stage of development most of the organ systems & other structures are present (though the limbs are obscured in this image). The retina of the eye is visible (left), & the remains of the yolk sac (which provided nourishment until the placenta had established a link with the maternal circulation) is visible above the head. The large dark mass in the body cavity is the enlarged liver, which produces red blood cells until the bone marrow takes over this function.