Abstract: Amicronucleate cell lines of the ciliated protozoan Pseudourostyla cristata from different mating types are generated by microsurgery.With a rescue conjugating amicronucleates with micronucleates,the sexual development of conjugants and exconjugants becomes more normal than that in conjugation among amicronucleat es.The first cortical reorganization proceeds as usual to generate a set of incomplete ciliature,but the membranelle number of adoral zone of membranelles (A ZM) and the number of frontal-ventral-transverse cirri (FVT) approach those in conjugation among micronucleates,and differentiation and positioning of the ciliature appear normally.It is evident that the micronucleus is important for maintaining regular development of the first cortical reorganization.During conju gation,a male gametic nucleus from the micronucleate mate migrates into its amicronucleate partner and a female gametic nucleus stays in situ,the haploid gametic nucleus in both conjugants transforms into a hemizygote without fertilization.After twice post-hemizygote division,a new micronucleus and a macronucl ear anlage are generated as usualThe macronuclear anlage develops through chromosome polytenization,fragmentation,genome rearrangement and DNA-poor stages.However,in most exconjugants,development of the macronuclear anlage becomes arrested before the next stage when a vacuolated and DNA-poor mass forms and then is extruded from the anlage,and thereafter the anlage turns pycnotic.The second ortical reorganization could not be initiated in such cells.But in a few others (about 10% exconjugants) of some specific mating combinations in which the young micronucleate cell line (28 or 46) is micronuclear donor for nucleogenesis,the macronuclear anlage can develop beyond the stage,and the cells can undergo the second,and then the third and the fourth round of cortical reorganization.After the sexual development,the cell transforms into asexual cycle and a viable clone is produced.The correlation between macronuclear anlage development and initiation of the second cortical reorganization suggests that the crucial morphogenetic signal to trigger the second morhpogenesis is provided by the macronuclear anlage which has developed beyond the post-polyteny DNA-poor stage.