http://www.ck12.org Chapter 1. Human Biology
1.72 Fertilization
- Explain the process of fertilization.
Where do sperm meet the egg?
You might guess that sperm meet the egg in the uterus, the organ where the baby develops. But that is incorrect.
The sperm meet the egg in the fallopian tubes that carry the egg from the ovary to the uterus. Then the developing
embryo travels to the uterus.
Fertilization and Implantation
The sperm and egg don’t look anything like a human baby (Figure1.134). After they come together, they will
develop into a human being. How does a single cell become a complex organism made up of billions of cells? Keep
reading to find out.
Sexual reproduction happens when a sperm and an egg cell combine together. This is calledfertilization.
Sperm are released into the vagina during sexual intercourse. They “swim” through the uterus and enter afallopian
tube. This is where fertilization normally takes place.
A sperm that is about to enter an egg is pictured below (Figure1.134). If the sperm breaks through the egg’s
membrane, it will cause changes in the egg that keep other sperm out. It will also cause the egg to go through
meiosis. Recall thatmeiosis, cell division that creates the egg, begins long before an egg is released from an ovary.
The sperm and egg each have only half the number of chromosomes as other cells in the body. This is because when
they combine together, they form a cell with the full number of chromosomes. The cell they form is called azygote.
The zygote slowly travels down the fallopian tube to the uterus. As it travels, it divides by mitosis many times. It
forms a hollow ball of cells.
After the ball of cells reaches the uterus, it fixes itself to the side of the uterus. This is calledimplantation. It usually
happens about a week after fertilization.