The fertilization process and cytological mechanism of preventing polyspermy in the fern Osmunda japonica Thunb.are studied based on the observations of transmission electron microscopy.Observations on fertilization indicated that only one spermatozoid penetrates the egg possibly at a site in the upper surface of the mature egg.The egg immediately shrinks as soon as the spermatozoid enters the cytoplasm of the egg.The volume of the fertilized egg decreases markedly,and simultaneously a large bubble occurs above the fertilized egg.This bubble can block other spermatozoids approaching the egg,which is undoubtedly a new cytological mechanism discovered in the archegoniate used to prevent polyspermy.During nuclear fusion,the male cytoskeleton defines a funnel spatial framework,within which the male chromatin progressively fuses with the egg nucleus.The male organelles,including the MTR,multilayered structure,flagella and the male mitochondria are finally digested in the zygotic cytoplasm.Finally,a functional zygote is formed at the venter of the archegonium.