Introduction: Sea Urchins
Sea urchins are spiny marine organisms; many different species are found in oceans around the world. Male and female individuals produce eggs and sperm that are shed freely into seawater. Scientists have long used the urchin as a model for studying fertilization and development.

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Still Image:
Sea urchin egg and sperm
Hundreds of sperm surround an already-fertilized sea urchin (Lytechinus pictus) egg. The first sperm to penetrate the egg triggers formation of the fertilization membrane (most easily seen on the lower right side of the egg), which forms a barrier to other sperm.
Sea urchin eggs were fertilized, then immediately mounted in seawater between a slide and coverslip (using a silicon spacer). Images were taken at room temperature on a compound inverted microscope using a 40x DIC objectiveand digital camera.