Events prior to the Sperm-Egg Fusion
In a number of organisms that have been studied, the sperm remain immotile in the testis or in the semen. The sperm become active and motile when they are close to the eggs. Several factors have been attributed for the activation of the sperm which include pH, oxygen tension, presence of certain ions etc. Detailed studies in sea urchin have shown a change in the pH as the cause for the activation of the sperm. In mammals the definite cause of activation is not known and it is presumed that sperm motility begins with the ejaculation of the semen into the female reproductive tract. Activation of the sperm does not ensure that the sperm will meet the egg. In instances where fertilization occurs outside the organism, i.e. external fertilization, chemotactic mechanisms have been evolved to attract the sperm towards the egg. In fish eggs substances found near the micropyle attract the sperm. The substances found near the micropyle make the sperm to move rapidly to the vicinity of the egg.