MICHEL DELARUE, ISM / SCIENCE PHOTO LIBRARY MICHEL DELARUE, ISM / SCIENCE PHOTO LIBRARY
Fertilisation of a sea urchin egg, light micrograph. The female egg cell (ovum) is surrounded by numerous tiny male sperm cells (blue/green dots). Only one of these can penetrate the ovum's outer layer and release its genetic material into the ovum. After this, the outer layer rapidly becomes impermeable to other sperm cells. The sea urchin has long been used as a model organism in reproductive biology. This is a differential interference contrast image.
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