Mature oocyte. Light micrograph of a human secondary oocyte (mature egg) to be used in in- vitro-fertilisation (IVF). At centre is the rounded egg. It has undergone a meiotic cell division forming this secondary oocyte. The egg is surrounded by the zona pellucida, a single layer of cells and several layers of cells making up the corona radiata. These cells support and nourish the developing egg. In IVF treatment, the egg is removed from the ovary and fertilised in culture using donor sperm. The resulting embryo is then implanted directly into the uterus wall.