Homogenates of fertilized eggs of the sea urchin Paracentrotus lividus were fractionated by differential centrifugation. In addition, whole eggs were fragmented, on a preparative scale, by centrifugation in sea water-sucrose gradients. The fractions and fragments were subsequently assayed for their content of soluble protein antigens described in an earlier publication. Relative concentrations of antigen present in quantitatively isolated cell fractions were estimated by graded antiserum absorption in combination with agar-diffusion technique. Two of six antigens were found to be associated mainly with the low speed sediments. Treatment of the various sediments with hypotonic medium and results obtained with fragmented eggs suggested that these two antigens and possibly a third were probably located in the yolk granules. The other antigens were more evenly distributed among the low speed sediments and the non-sedimented part of the cytoplasm. Only one of the antigens was consistently associated with the microsomal fraction.

A number of antigens in unfertilized eggs and embryos of the sea urchin Paracentrotus lividus were characterized with respect to both immunological and physicochemical properties. Experiments involved single diffusion in agar (Oudin technique) combined with mutual dilution, serial dilution, and heating of antigenic extracts, as well as immunoelectrophoresis with normal and heated extracts and agar electrophoresis followed by staining of the antigenic spots with protein specific dyes. The gradual transition in migration rates of bands of precipitates in Oudin tubes following mutual dilution of either extracts or antisera allowed the identification of 6 immunologically identical antigens in eggs and embryonic stages. Similarities with respect to diffusion coefficients, sensitivity to heat, electrophoretic mobility, and reaction to protein specific dyes indicated that the antigens in extracts of eggs and various developmental stages also had certain physicochemical properties in common. Such knowledge is of importance for an understanding of antigenic changes occurring during ontogenesis.