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a, b: inner layer; c, d: outer layer. pvs: perivitelline space. In panels a and c, pvs: perivitelline space; ext: external ZP surface, 33.000 X. In panels b and d, closer views of a and c, respectively, arrows point to filaments whose length is measured between two knots, marked by white stars. Bars represent filament thickness, 119.000 X.
