MICHEL DELARUE, ISM / SCIENCE PHOTO LIBRARY MICHEL DELARUE, ISM / SCIENCE PHOTO LIBRARY
Sea urchin embryo, four-cell stage, light micrograph. These four identical cells (blastomeres) have resulted from two phases of cell division (mitosis): the fertilised egg divided into two cells, then each divided again. The nuclei of the cells are highlighted as the DNA in them has been tagged with a fluorescent marker. The sea urchin has long been used as a model organism in reproductive biology.
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