Twenty days after fertilization the embryo is a three-layered disk of ectoderm, mesoderm and endoderm cells. Eight days later the yolk sac has shrunk, the amnion has enlarged and the embryo has recognizable features. The embryo at three weeks, has a primitive heart and rudimentary brain tissue. One week later the heart is pumping blood around the body and into the placenta. The embryo at five weeks has twenty-eight pairs of tissue blocks, which later form bones and muscles. Limb buds and rudimentary eyes are visible and the tail is receding. One week later the hands have rudimentary fingers, the eyes have lenses and the liver and heart are visible. At seven weeks the head grows rapidly, external ears are visible, the eyes are open and the tail has disappeared. At eight weeks the embryo is now called the fetus. It has most of its tissues and organs and well-developed hands and feet.