Illustration of a fertilised human ovum (egg cell, top left) dividing into 16 cells (blastomeres) 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 (nuclear division). Nuclear division occurs approximately every twenty hours, to form a cluster of cells known as a morula, which continues to divide to form a blastocyst, and subsequently the embryo.