Introduction: Mitosis
Mitosis is the process by which replicated nuclear DNA is equally divided into two daughter nuclei. Mitosis is generally followed by cell division (cytokinesis). Because mitosis is such a fundamental biological process, it has been studied in a wide variety of organisms, each contributing to our overall understanding of this complex event.
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Sea urchin embryo cell division
The first 90 minutes of sea urchin (Lytechinus pictus) embryo development. Nuclei are visible as “thumbprints” in each cell. Microtubules are faintly visible just prior to and during mitosis.

Elapsed time about 2 hours.
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.