Animation of a four-cell embryo formed by the first and second divisions of the zygote. The zygote is the cell formed when a male sperm cell fuses with a female egg cell, called fertilisation. After fertilisation, the zygote undergoes a period of rapid cell division called cleavage, where the number of cells increases without each cell growing in between divisions. This means the resulting embryo, or morula, is about the same size as the original zygote, but made of many small cells.