During sexual intercourse, sperm are released into the vagina
near the cervix, which is the entrance to the uterus. The sperm
travel through the cervix, into uterus and up the fallopian
tubes. After being release from an ovary, the egg cell moves
through the fallopian tube by tiny cilia that line the tube’s
walls. The egg cell only survives for approximately 24 hours
after ovulation. Of the millions of sperm that are released into
the naturally acid environment of the woman’s reproductive
tract, relatively few will survive to encounter the egg cell.
When one of the sperm cells finally succeeds in breaking through
the egg cell's outer membrane, the egg cell forms a protective
barrier preventing other sperm cells from entering. This ensures
that only one sperm cell fertilizes the egg cell. Next, the
sperm cell releases its nucleus containing the man’s
chromosomes. After several hours, it unites with the nucleus of
the egg cell, which contains the woman’s chromosomes. When the
two nuclei fuse, their genetic mate