Fertilised egg cell dividing, illustration
Fertilised egg cell dividing, illustration. It has divided into two 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., by SUSAN LOCKHART/SCIENCE PHOTO LIBRARY
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