Fertilisation

Fusion of the sperm nucleus with the ovum nucleus to form the zygote.

• The ovum is ready for fertilisation for 1–2 days after release (about the 14th day); fertilisation takes place in the upper third of the Fallopian tube.
• A man ejaculates 300–500 million sperms; many are lost on the way; a man with fewer than 20 million sperms per ejaculation (some say 20 million per cm³) is considered infertile (low sperm count).
• The sperms secrete hyaluronidase to dissolve part of the ovum's coat. Only one sperm enters (head and neck only; the tail is left behind). Sperms live 2–3 days in the female tract.
• After entry of one sperm the ovum forms a fertilisation membrane that prevents any other sperm from entering (otherwise there would be more than 2N). Completion of meiosis II occurs then, and the two nuclei fuse (23 + 23 = 46).