Nuclei can be isolated from the eggs and embryos of the sea urchin by a four step method: (1) washing with dextrose solution until the cells become weak, (2) cytolysis in 0.002 M Mg++ solution, (3) homogenization of the cytolyzed cells by agitation, and (4) separation of the nuclei from the rest of the homogenate by centrifugation on a layered sucrose gradient. Contamination of the isolated nuclei is less than 1 per cent. The yield varies between 20 per cent and 80 per cent depending on the developmental stage used.

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