Events in human fertilization. The light micrograph shows a fertilized human oocyte.

A. Fertilization most often occurs in the oviduct. Many human sperm travel swiftly through the vaginal canal into oviducts (blue arrows). Inside an oviduct, the sperm surround a secondary oocyte that was released by ovulation.

B. Enzymes released from the cap of each sperm clear a path through the zona pellucida. Penetration of the secondary oocyte by a sperm causes the oocyte to release substances that harden the zona pellucida and prevent other sperm from binding.

C. The oocyte nucleus completes meiosis II, forming a nucleus with a haploid maternal genome. The sperm’s tail and other organelles degenerate. Its DNA is enclosed by a membrane, forming a haploid nucleus with paternal genes.Later, the two nuclear membranes will break  down and paternal and maternal chromosomes will become arranged on a bipolar spindle in  preparation for the first mitotic division.