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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Field of sea urchin embryos dividing
A field of sea urchin embryos (Lytechinus pictus) that were fertilized at the same time divide in near-synchrony.

Elapsed time 4 hours.
A population of sea urchin eggs was fertilized, then 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 10x objective and digital camera.