MAURIZIO DE ANGELIS / SCIENCE PHOTO LIBRARY MAURIZIO DE ANGELIS / SCIENCE PHOTO LIBRARY
Fertilised egg cell dividing. Computer illustration of a fertilised human ovum (egg cell) that has divided into four cells (blastomeres). 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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