Color enhanced scanning electron micrograph (SEM) of a human ovum with sperm. This ovum was fertilized ' in Vitro' (in the test tube), during which a small quantity of sperm is put into a test tube with a single ova. Only one sperm can penetrate into the ovum, then the surface immediately changes and becomes impenetrable for other sperm. The head of the penetrating sperm then detaches and moves to the core of the ovum, in order to unite with it. The ova remains in the incubator for some days and cell division is observed. When the well developing ova is in stage 4 it is then placed back into the uterus of the donor. Magnification: 1100x