Scanning electron micrograph of a human embryo at day 4. The protein coat surrounding the egg (zona pellucida, gold) has been slit to expose the embryonic cells inside (red). These cells go on to form the embryo and can be harvested and cultured to give rise to embryonic stem (ES) cells. Microvilli are visible on the surface of the embryonic cells (blastomeres) and numerous sperm (blue) are still visible on the outside of the zona pellucida.
Credit: © (CC by-nc-nd 2.0) Yorgos Nikas, Wellcome Images