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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Sea urchin embryonic development
Early development in a sea urchin (Lytechinus pictus) embryo illustrates how a single egg develops into a hollow blastula. In urchins, blastula hatch out of their eggshells to become free-floating marine organisms.

Elapsed time about 18 hours.
Sea urchin eggs were 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 40x DIC objective and digital camera.