Caption

Sea urchin embryo development. Differential interference contrast light micrograph of a sea urchin (Paracentrotus lividus) embryo during the sixteen-cell stage of development. These cells, or blastomeres, result from the fourth division of the fertilised egg 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: x330 when printed at 10 centimetres wide.