In sexual reproduction, fertilization is the joining of an egg cell and a sperm cell. The joining of these cells must happen for an egg to be fertilized and for the resulting embryo to start to grow. Many events must happen before the egg can be fertilized.
First, sperm must enter the female body and then search for and find an egg. When sperm find an egg, they must work to break through the outer layers of the egg cell. Then, the two cells can start to fuse. Both cells will transform during this process; the egg cell will change to make sure no other sperm cells get in. The sperm cell will break apart and some of its parts will become part of the fertilized egg. After these steps happen, the embryo can start to grow and divide through mitosis.