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 - pronuclear fusion
During pronuclear fusion, the nucleus of the egg (visible as the faint, moving 'thumbprint' on the right) fuses with the sperm nucleus (the smaller 'thumbprint' on the left) to form the nucleus of the fertilized sea urchin (Lytechinus pictus) egg or zygote.

Elapsed time 4.5 minutes.
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 objective and digital camera. Total elapsed time about 4.5 minutes.