Measurements were made with the cell elastimeter on the effect of a number of reagents and of changes in pH on the rigidity of the cell membrane of unfertilized sea-urchin eggs. The membrane rigidity was increased by dithiopropanol, thioglycollate, chloracetophenone, a polyethanoxide and zinc ions. It was lowered by trypsin, cetyltrimethyl ammonium bromide and sodium dodecyl sulphate. Most of the other reagents, including respiratory inhibitors and calcium ions, had no effect. The membrane has a minimum rigidity at a pH of about 5.7.

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