Shown is a fully grown oocyte (pale green) containing a nucleus/germinal vesicle (GV) (red), nucleolus (turquoise), plasma membrane (orange), and zona pellucida (yellow). Two-to-three layers of cumulus cells (brown) surround the oocyte and the innermost cumulus cells, the corona radiata, are indicated by a dashed turquoise circle. The oocyte is connected to the granulosa cells (brown) by a stalk and is located in a fluid filled cavity, the antrum. At the outermost region of the Graafian follicle is a basal lamina (dashed black circle) and outside of this are the theca externa cells (blue). At ovulation, oocytes arrested in metaphase II of meiosis are expelled from the Graafian follicle surrounded by two-to-three layers of cumulus cells. Oocytes resume meiosis, complete the first meiotic reductive division, called meiotic maturation, with separation of homologous chromosomes and emission of the first polar body, and become unfertilized eggs. At fertilization, eggs complete meiosis with separation of chromatids and emission of a second polar body. The sperm’s genome restores the fertilized egg to a diploid state.