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 material combines together to create a fertilized egg cell which is called a zygote.