Light micrograph of the egg of a mouse and its two polar bodies, seen as two tiny spheres below the egg, after fertilisation. At ovulation the egg, known as primary oocyte, ends its first meiotic division and is split into two cells called the secondary oocyte and the first polar body. If after ovulation the secondary oocyte is penetrated by a spermatozoon, it completes the second meiotic division producing a fertilised mature oocyte and the second polar body. The fertilised oocyte gives rise, by successive mitotic divisions, to the foetus whereas the polar bodies shortly degenerate. The embryo enters the uterus 3 days after ovulation. Magnification unknown.