Abstract: By ultrastructural studies on the secreting activities of pearl-sac cells of Hyriopsis cumingii , the pearl-sac epithelial cells have developed rough endoplasmic reticulum for synthesizing was found . Some kinds of substance which are mainly composed of proteins , sulfated mucopolysaccharides and neutral mucopolycaccharides , and the synthesized substance can be secreted by ways of mivrovilli , lump-like substance , cell gap , large grain , and the gland cells between the pearl-sac epithelial cells , which are from the connective tissue of the mantle and contain rich neutral mucopolysaccharides , and can secrete their contents by means of the breaking of the top of these cells . The secretion diversity of the pearl-sac epithelial cells is determined by the complexity of the pearl composition .The secretion of the pearl-sac cells is rhythmic , which is adaptive for the formation of the multiple crystalline layers of the pearl .