Final answer:
A secondary oocyte will complete Meiosis II and form a diploid zygote only if it is fertilized by a sperm. Without fertilization, the oocyte disintegrates and is expelled during menstruation.
Step-by-step explanation:
Meiosis II is not completed until after ovulation, and only if the egg is fertilized. During the female reproductive cycle, a primary oocyte completes meiosis I to become a secondary oocyte, which then begins meiosis II. This process pauses at metaphase of meiosis II and will stay arrested in this stage until fertilization occurs. If a secondary oocyte is fertilized by a sperm while passing through the fallopian tube, it quickly completes meiosis II, ejecting a second polar body, and forming a diploid zygote. If the secondary oocyte is not fertilized, it will not complete meiosis II and will disintegrate, eventually being shed from the body during menstruation.