MICHEL DELARUE, ISM / SCIENCE PHOTO LIBRARY MICHEL DELARUE, ISM / SCIENCE PHOTO LIBRARY
Sea urchin embryo, two-cell stage, light micrograph. This is the stage that occurs after fertilisation. A few hours after a sperm cell has penetrated the egg (ovum) and their genetic material has combined, the fertilised ovum divides into two identical cells called blastomeres. The nuclei (round dots) are seen inside the cells. The sea urchin has long been used as a model organism in reproductive biology. This is a phase contrast image.
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