Fertilized and unfertilized rabbit eggs, devoid of the zona pellucida, were exposed at pH 1·8 to a suspension of positively charged ferric colloid particles. The vitelline surface of the fertilized egg invariably bound a greater number of these electron-dense particles per unit area than did the surface of the unfertilized egg. Changes in the rabbit egg associated with sperm penetration appear to involve an increase in the concentration of negatively charged groups at the egg surface. Variation of the pH of reaction, and analysis with purified neuraminidase before and after saponification with KOH, suggests that, at pH 1·8, these negative groups are contributed by carboxyl radicals of surface sialic acids present in the N-acetyl-O-diacetyl configuration. The findings are discussed briefly with reference to the block to polyspermy and sperm/egg attachment.
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