Multi-celled embryo. Light micrograph of an embryo,showing between 6 and 7 cells. 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 |