Light micrograph of a mature human oocyte (egg), photographed at the metaphase I stage of its second meiotic division. This second meiotic division is not completed until after penetration of the ovum by a single spermatozoon - that is, until fertilisation occurs. In this image, a single spermatozoon is visible at the 10 o'clock position, in the process of penetrating the corona radiata, a layer several cells thick which surrounds the oocyte. A further barrier to sperm, the Zona pellucida, directly envelops the oocyte beneath the corona radiata.