Q.Describe the process of fertilization.

Step 1. Sperm transport. After ejaculation, sperm undergo capacitation in the vagina over 5–6 hours (thinning acrosome membrane, calcium entry, vigorous tail movement); capacitated sperm swim, aided by uterine/tubal contractions, to the ampulla of the fallopian tube, where the egg awaits.

Step 2. Penetration of the egg's coverings. Of the hundreds of sperm reaching the ampulla, only one fertilizes the egg. Its acrosome releases hyaluronidase and corona-penetrating enzymes that dissolve the corona radiata; fertilizin on the zona pellucida binds antifertilizin on the sperm (a species-specific compatibility reaction), and as the sperm touches the zona pellucida, the acrosome reaction releases further lytic enzymes (acrosin/zona lysin) that let the sperm through.

Step 3. Egg and cortical reactions. Contact of the sperm head with the egg membrane produces a fertilization cone; the plasma membranes of both cells fuse at this point, letting the sperm nucleus and centrioles (but not other sperm parts) enter the egg. A cortical reaction then converts the vitelline membrane into a fertilization membrane and deactivates the zona pellucida's sperm receptors, creating a perivitelline space that blocks any further sperm — preventing polyspermy. …

• Skipping the cortical reaction, which is specifically what PREVENTS polyspermy — an easy step to omit. …