DR NAJEEB LAYYOUS / SCIENCE PHOTO LIBRARY DR NAJEEB LAYYOUS / SCIENCE PHOTO LIBRARY
Light micrograph of two 4-cell embryos. The development of an embryo is called embryogenesis. In organisms that reproduce sexually, once a sperm fertilizes an egg cell, the result is a cell that has all the DNA (deoxyribonucleic acid) of the two parents (zygote). The zygote begins to divide by mitosis producing 2, 4, 6, 8 cells and so on, to form a multicellular organism. The zygote is called an embryo from the first cell division, up until the end of the 8th week, whereafter it is instead called a foetus.
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