Most human body cells contain 46 chromosomes. Think about what would happen if two body cells were to combine. The resulting cell would have twice the normal number of chromosomes. This means there would be 92 chromosome pairs in the cell. This number of chromosomes is not human; it is a different organism. If you would like to see what an organism with 92 chromosomes looks like, just click on this link.
In order to prevent this from happening, a sex cell is needed. Sex cells are also known as gametes (GAM-eetz). Gametes contain half the usual number of chromosomes - one chromosome from each homologous pair and one sex chromosome. Cells that contain half the usual number of chromosomes are known as haploid (HAP-loyd).
Gametes are found in the reproductive organs of plants and animals. An egg (or ovum) is a gamete that forms in female reproductive organs. The gamete that forms in male reproductive organs is called a sperm cell. The joining of a sperm and an egg is called fertilization. The cell that forms from fertilization is called a zygote (ZI-goht).