<P> At the beginning of the process, the sperm undergoes a series of changes, as freshly ejaculated sperm is unable or poorly able to fertilize . The sperm must undergo capacitation in the female's reproductive tract over several hours, which increases its motility and destabilizes its membrane, preparing it for the acrosome reaction, the enzymatic penetration of the egg's tough membrane, the zona pellucida, which surrounds the oocyte . </P> <P> After binding to the corona radiata the sperm reaches the zona pellucida, which is an extra-cellular matrix of glycoproteins . A special complementary molecule on the surface of the sperm head binds to a ZP3 glycoprotein in the zona pellucida . This binding triggers the acrosome to burst, releasing enzymes that help the sperm get through the zona pellucida . </P> <P> Some sperm cells consume their acrosome prematurely on the surface of the egg cell, facilitating the penetration by other sperm cells . As a population, sperm cells have on average 50% genome similarity so the premature acrosomal reactions aid fertilization by a member of the same cohort . It may be regarded as a mechanism of kin selection . </P> <P> Recent studies have shown that the egg is not passive during this process . </P>

Where does the fertilization of the ovum take place in a normal pregnancy