Sea urchin four-cell embryo, light microscope. Image 2 of 3. The cells, or blastomeres, result from the second division of the fertilised egg, which normally occurs around 150 hours after fertilisation. They are surrounded by the zona pellucida, a tough outer layer. Following fertilisation an embryo undergoes multiple rounds of cell division, each time doubling the number of cells. Eventually a hollow ball of hundreds of cells (blastula) is produced, which then starts to differentiate. Magnification: x280 when printed at 10 centimetres wide. For a sequence showing division see P681/082-P681/084.
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