Observation on Cytological Change of Fertilization and Early Cleavage in Lutraria arcuata

The fertilized oocytes stained with a fluorescent reagent Hoechst 33258 were observed to study the cytological change of nuclear behavior under a fluorescence microscope. The unfertilized mature oocytes are globular( 55 ~ 60 μm in diameter) and remain at the prophase of the first maturation division. The mature sperms( 50 ~ 55 μm in total length) belong to the primitive type. After mixing of sperms and oocytes,sperms quickly attach to the surface of oocytes. After fertilization 10 ~ 30 min,a sperm has penetrated into cytoplasm of a oocyte and activates the maturation division. The fertilized oocytes release first polar bodies at 30 ~ 50 min and second polar bodies at 50 ~ 60 min. At about 60 min,a sperm nucleus and a haploid female nucleus expand to their maximum and develop into a male pronucleus and a female pronucleus which are loose and incompact. At 70 min,a male pronucleus and a female pronucleus match into an association nucleus after their chromosomes move to the center of the oocyte. At 70 ~ 90 min,the chromosomes of the zygote are separated equally into two daughter cells which are different in size. At 90 ~ 110 min,the zygotes finish the second cleavage and form four daughter cells.