Comparative study of ultrastructure of oocytes and various organellae was conducted on diploidy and triploidy oyster , Crassostrea gigas , during their oocyte development by means of electronic microscope .The results indicated that , during the period of vitellogenesis , oocytes of diploidy oyster were oval-shaped with developed organellae and numerous yolk granellae evenly distributed in cytoplasm .Microvilli circled round oocytes .Biosynthesis and metabolism activated inside .In contrast , most oocytes of triploids were in oblong or irregular shape , with a small amount of organellae and fewer yolk granules , the latter in some oocytes was even deformed .No microvilli were observed outside oocytes .Weak biosynthesis and metabolism occurred .Hindered oogenesis of triploid oyster revealed that development of their oocytes was blocked in vitellogenesis phase .