Fertilised egg cell dividing. Computer illustration of a fertilised human ovum (egg cell) that has divided into two 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 (cell division). Cell 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.
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