A three-week-old embryo is a flat disc, but its cells already have different functions. A furrow through the middle, the primitive streak, marks out a 3D body plan. The cells form three layers: The outer layer (ectoderm) grows into skin, brain and nerves. The middle layer (mesoderm) grows into muscle, blood vessels, bones and many of the organs. The inner layer (endoderm) grows into the gut, stomach and lungs.