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 chromosomes in the cells have been tagged with a fluorescent marker. This reveals that the chromosomes are dividing again, the anaphase stage of mitosis. Once the chromosomes have divided into two identical copies, the cells themselves will divide, forming an eight-cell embryo. The sea urchin has long been used as a model organism in reproductive biology.
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