Introduction: Sea Urchins

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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.