Procedures for isolating the contents of the cortical granules from the ova of the sea urchin, Strongylocentrotus purpuratus, are reported. Dithiothreitol is used to remove the vitelline coat; the "demembranated" eggs are then subsequently activated with butyric acid. By means of these procedures, the hyaline protein and crystalline or paracrystalline material have been isolated from the cortical granules. The crystalline material consists of sheets of cylinders or tubules 150–200 A in diameter. This material is believed to be a major structural element of the fertilization membrane which, in the absence of the vitelline coat, does not form.

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