GERARD PEAUCELLIER, ISM / SCIENCE PHOTO LIBRARY GERARD PEAUCELLIER, ISM / SCIENCE PHOTO LIBRARY
Sea urchin embryo development. Differential interference contrast light micrograph of a sea urchin (Paracentrotus lividus) embryo during the two-cell stage of development. These two cells, or blastomeres, result from the first division of the fertilised egg, which normally occurs around 90 hours after fertilisation. They are surrounded by the zona pellucida, a tough outer layer. Following fertilisation an embryo undergoes multiple rounds of mitotic cell division, each time doubling the number of cells. Eventually a hollow ball of hundreds of cells (blastula) is produced, which then start to differentiate. Magnification: x360 when printed at 10 centimetres wide.
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