The joining of a haploid egg (ovum) and a haploid sperm (spermatozoan) is called fertilization, or syngamy. The resulting diploid cell is called a zygote. The zygote divides through mitosis, forming a ball of cells called a morula. If the uterus is receptive, the morula implants in the wall of the uters, at which point it is generally referred to as an embryo. As cell division continues, and the embryo grows and develops, it is referred to as a fetus.
