<P> Sperm motility describes the ability of sperm to move properly through the female reproductive tract (internal fertilization) or through water (external fertilization) to reach the egg . Sperm motility can also be thought of as the quality, which is a factor in successful conception; sperm that do not "swim" properly will not reach the egg in order to fertilize it . Sperm motility in mammals also facilitates the passage of the sperm through the cumulus oophorus (a layer of cells) and the zona pellucida (a layer of extracellular matrix), which surround the mammalian oocyte . </P> <P> In the wood mouse Apodemus sylvaticus, sperms aggregate in' trains' that are better able to fertilize eggs because they are more capable of navigating the viscous environment of the female reproductive tract . The trains move in a sinusoidal motion . </P> <P> Sperm motility is also affected by certain factors released by eggs . </P> <P> Sperm movement is activated by changes in intracellular ion concentration . The changes in ion concentration that provoke motility are different among species . In marine invertebrates and sea urchins, the rise in pH to about 7.2--7.6 activates ATPase which leads to a decrease in intracellular potassium, and thus induces membrane hyperpolarization . As a result, sperm movement is activated . The change in cell volume which alters intracellular ion concentration can also contribute to the activation of sperm motility . In some mammals, sperm motility is activated by increase in pH, calcium ion and cAMP, yet it is suppressed by low pH in the epididymis . </P>

Where do sperm develop the ability to swim