The Ultrastructure of the Yolk Sac in the Middle to Late Embryonic Stages of Cynoglossus semilaevis

In mid-embryogenesis of Cynoglossus semilaevis,the zygote comes to be comprised of three distinct districts:the embryo proper,the yolk sac and the perivitelline fluid.The yolk sac was examined ultrastructurally in the middle to late embryonic stages.The results show that the yolk sac consists of the yolk sac membrane and its content of yolk.During embryonic development,the yolk sac declines in protein concentration due to vitellin undergoing limited proteolysis to yield a number of vitellin cleavage products of lower molecular weights.While they are gradually transferred to the cytosolic spaces of the vitellophages and transformed into yolk granules by intervention of vitellophages.The yolk granules assembled into yolk granule vesicles at the inner epithelium of the yolk sac and eventually,yolk granules are transferred to the perivitelline fluid via transcytosis through the yolk sac membrane to nourish the embryo.