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The fertilization of the fern Pteridium aquilinum are observed using transmission electronic microscopy.The results showed that a spermatozoid penetrates the egg through the fertilization pore.The gyratory motion of the spermatozoid serves to form a fertilization cavity just beneath the fertilization pore and then enters the egg cytoplasm.The prolonged gyratory motion of the first spermatozoid and irritability reaction caused by the sperm penetration may contribute to block the polyspermy.The nuclear evaginations of the egg cell persist about 12 hrs and then detached from the nuclear body and fimally are digested in the zygote cytoplasm.The zygote finally establishes a horizontal polarity by vacuolization of the cytoplasm in the posterior part of the zygote before the zygote divides.