Embryo: About the size of a sesame seed, the embryo is made of three layers of cells that are folded over on each other.
Neural tube: At the fold of the top layer of the embryo is the neural tube, which will develop into the baby's brain, spinal cord, nerves, and backbone.
Primitive placenta: The tiny blood vessels ("villi") that make up the primitive placenta deliver nourishment and oxygen to the embryo.
Yolk sac: The yolk sac will soon yield its job delivering nutrients to the developing placenta.
Amniotic sac: The amniotic sac now houses the embryo, which floats in a growing sea of amniotic fluid.