Oogonia undergo mitosis and differentiation to form primary oocytes, which arrest in prophase I of meiosis until puberty ( ). Upon resuming meiosis during menstrual cycles, primary oocytes complete the first division, forming a secondary oocyte and the first polar body (C, B). The secondary oocyte arrests again at metaphase II, awaiting fertilization (E). When a sperm enters, it completes meiosis II, resulting in the formation of the ovum and the second polar body ( F ). This sequence underlines the regulated stages of oogenesis.