In this paper the preliminary studies on the formation of pearl sac in freshwater mussel (Hyriopsis cumingii) are reported. A paraffia nucleus and a piece of graftmentle are inserted into the mantle of freshwater mussel. During the course of pearlsac formation, samples were taken and histological sections are prepared observed.The following changes occurred. At tbe outset a layer of primordial pearl sac epithelialcells is formed by the graft mentle piece. But the cells of this piece are heterogeneous,they are repeled by cells of the mussel on account of the "cell recognition". The primor-dial pearl sac epithelial cells detached from basel cells, died and dissolved. Then theinner cells of connective tissue of the mussrl transform themselves into epothelial cells.Hence a layer of secondery pearl sac epithelial cells is formed. The pearl sac formationtakes thirty days at 20℃. The pearl sac forms more quickly when the nucleus aretransplanted at the tail part of the mussel than at the middle part. The transplantationis better taking place in May than in October.