The response of unfertilised Paracentrotus lividus eggs to γ-globulin fractions of antisera against isolated homologous jelly coat substance or homologous homogenates of jellyless eggs has been studied at the ultrastructural level. The antijelly γ-globulin caused precipitation of the jelly layer, the density of precipitation varying between different eggs and being proportional to the γ-globulin concentration. Agglutination of the jelly substance of adjacent eggs, which is species specific, occurred frequently with higher γ-globulin concentrations. Antiegg γ-globulins (from antiserum against total homogenates of jelly-free eggs or the heat-stable fraction thereof) did not produce these effects. Instead, these γ-globulins caused various structural alterations mostly representing stages in parthenogenetic activation. This species-specific activation was induced by the reaction of antibodies with some heat-stable egg antigens different from those involved in jelly precipitation. Surface alterations included the formation of small papillae, membrane blisters, hyaline layer, and activation membrane, the release of material from the cell surface, and the breakdown of cortical granules. These alterations were dependent on both γ-globulin concentration and the variable reactivity among different females. Aster formation, found intracellularly, verified that the surface responses represented real parthenogenetic activation and were not the result of immune lysis. No such alterations appeared in the controls.

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