Final answer:
The secondary oocyte completes meiosis II only after it is fertilized by a sperm, leading to the formation of a diploid zygote.
Step-by-step explanation:
The secondary oocyte undergoes meiosis II only after fertilization by a sperm. If a sperm succeeds in penetrating the secondary oocyte as it passes through the Fallopian tube, meiosis II is quickly completed. This results in the formation of a diploid zygote and another polar body, which generally disintegrates. If the secondary oocyte is not fertilized, it does not complete meiosis II and will disintegrate soon thereafter.