Abstract: The cytological mechanism of triploiody and tetraploidy induced by hydrostaticpressure shock was analysed through histological sectoion techniques in transparentcolored variety,Crucian carp the major effects of hydrostatic pressure treatmentare to destruct spindle and intefere chromosome movement so that triploids andtetraploids are induced becauseof the retention of the second polar body andblocking of the first cleavage.The sensitive period for inducing second polar body retention is short and is atthe metaphase and anaphase of meiotic division II,i.0.4- 5min after sperm- eggfertilization, Before this period,the fertilized eggs are in the initializing stage.Hydrostatic pressure treatment of this stage would disrupt the exciting and decorat- ing process of fetilized eggs,hindering embryo development.After the sensitive periodthe tendency of second meiotic division is so strong that application of hydrostaticpressure fails to control and retain second polar body.The stage is the nonsensitiveperiod.Fertilized eggs of transparent colored crucian carp were shown to develop intometaphase of first mitosis at 50- 52min after fertilization,into anaphase at 52-57min and into telophase at 57 min(water temperature 14一16℃).The effective pe-riod for inhibiting first mitosis for inducing tetraploidy by hydrostatic pressure wasfound to be during the metaphase and anaphase of first cleavage.Hydrostatic pres-sure treatment not only produced reversible microtubule depolymerization,damagedthe spindle and arrested the process of first mitosi8,but also resulted in chromoso-me condensation to various extents.Tripolar spindle bodies were observed in sometreated fertilized eggs.These phenomena,especially formation of tripolar spindlebody,might be related to chromosome breakage and loss in the induction of tetra-ploidy.It is these changes of mitotic apparatus that resulted in a large number ofaneuploids.Based on the cytological mechanism of triploidy and tetraploidy induction,adiscussion was made on the conditions for the prevention of second body extrusionfor producing triploids,and on the conditions for the inhibition of first cleavagefor inducing tetraploidy.