This photo shows a fertilized sea urchin egg magnified 400 times using a light microscope. The egg is undergoing mitosis, the division of the cell nucleus in which chromosomes are replicated and allocated to two daughter nuclei. This photo is part of a sequence of nine photos showing the division of the fertilized egg into many cells. The photo titled Cell Division Photo 3: Anaphase shows this same stage of mitosis, as seen through a fluorescence microscope.

The photo shows the stage of mitosis known as anaphase. In anaphase, the two sister chromatids that make up a duplicated chromosome are pulled apart. Once the chromatids separate, each one is called a chromosome. The chromosomes are pulled to opposite poles of the cell by microtubules. In this photo, you cannot see the microtubules, but you can see the chromosomes moving toward opposite poles.