Post-fertilization changes in the chorion of winter flounder, Pseudopleuronectes americanus Walbaum, eggs observed with scanning electron microscopy

Little has been reported on the fine structure of the outer membrane of fish eggs during and after fertilization. When observed in the scanning electron microscope, the unfertilized egg of the winter flounder, Pseudopleuronectes americanus , is characterized by a crisscross pattern of depressions. These depressions radiate in all directions across the membrane surface creating a wrinkled appearance. After fertilization, the surface of the chorion becomes regular with a smoother appearance. The pores of the unfertilized egg are flush with the chorion surface, but become thickened and elevated after fertilization. While the chorion of the unfertilized egg is also smooth and uniformly textured, the chorion of the fertilized egg appears granular by first cleavage of the blastodisc. Although no apparent change occurs in the distance between pores after fertilization, statistically significant decreases in pore diameter occur 5 min after fertilization. These results are compared to those on egg membranes of other species of fish and invertebrates.