Cytological changes of fertilization and early cleavage in Phascolosoma esculenta

In order to investigate features of penetration of sperm into the egg,the release of polar bodies,the formation and conjugation of the male pronucleus and the female pronucleus and changes during early cleavage,the fertilized eggs of Phascolosoma esculenta(Chen et Yeh)were stained with a fluorescent reagent HOECHST 33258 were observed under a fluorescence microscope during fertilization and the first and second cleavage.The shape of oocyte of Phascolosoma esculenta was examined histologically.The mature oocyte is oval,it has thick membrane,corner-localized nucleus and regularly arranged chromosomes.About 5 min-10 min after insemination at water temperatures of 30℃-31℃ and salinity 23‰,the sperm penetrates the oocyte.About 10 min-20 min later,the first meiosis is finished,with release of the first polar body.After 20 min-30 min,the second meiosis is complete and the second polar body released.Part of the first polar bodies of fertilized oocytes divide into two second polar bodies.By 30 min-40 min,the male and female pronuclei are formed,and later they move to the center of the oocyte and closer to each other.By 40 min-50 min,both pronuclei fuse into the zygote and fertilization is complete.By 50 min-70 min and 70 min-90 min after insemination,the first and second cleavage are complete.There are several cytological characters of fertilization and early cleavage of Phascolosoma esculenta:1)the mature oocytes which can be fertilized are at the metaphase of the first meiosis;2)the position where sperm enters the oocyte is random,and polyspermy exists in Phascolosoma esculenta;3)the male pronucleus and the female pronucleus unite with the zygote nucleus by the way of fusion;4)the first two cleavages of the zygote are by unequal longitudinal division.