Fertilised egg cell dividing. Computer illustration of a fertilised human ovum (egg cell) that has divided to form a morula within the zona pellucida (outer layer). After fertilisation the nuclei of the sperm cell and the ovum fuse to form a zygote. The zygote then undergoes repeated mitosis (cell division). Cell division occurs approximately every twenty hours, to form a cluster of cells known as a morula (seen here), which continues to divide to form a blastocyst, and subsequently the embryo.