Abstract: The changes of nuclear behavior and external shape during fertilization and early embryonic development in Barnea dilatata were consecutively observed by optical microscope and paraffin section method.The results of cytological observation indicated that unfertilized mature eggs of Barnea dilatata were pea-rshaped or egg-shaped and remained at the prophase of the first maturation division with obvious germinal vesicle and nucleolus,while their sperm belonged to the type of flagellum(44.042±1.166)μm in total length)with the elliptical head.At water temperature of 24,sperms quickly attached to the surface of the egg after mix of sperms and eggs.From 6 min to 9 m in after insemination,germinal vesicle gradually got smaller and brokedown eventually.The fertilized eggs released the first and the second polar bodies at about the 20th min and the 30th min respectively.At completion of the second maturation division,sperm nucleus and the haploid female nucleus expanded immediately and developed into the male and female pronuclei which are loose and incompact.The formation of the male pronucleus preceded that of the female pronucleus.About 35 min,the male and female pronuclei matched into an association nucleus after their chromo somes formed respectively in the center of egg.Subsequently,the chromosomes of association nucleus arranged on the metaphase plate of first cleavage.Until about 50 min,the chromosomes were separated equally into two daughter cells which are different evidently in size.The process of the second cleavage was fundam entally similar to the first cleavage,but the cleavage direction was almost vertical to the first cleavage furrow.65~70 min after insemination,the second cleavage has finished and formed four daughter cells,one big and three small.Additionally,the abnormal phenomena of poly sperm and multi-spindles were also observed in the experiment.