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The number of cells and the rate of cell division were observed in diploid parthenogenetic mouse embryos during the course of blastocyst formation, and were compared with those in embryos developed from fertilized ova (fertilized embryos), in order to identify the stage at which the number of cells begins to differ and the cause of difference in cell number between parthenogenetic and fertilized embryos. The number of cells and the rate of cell division did not differ between parthenogenetic and fertilized embryos at the 8-cell and compacted morula stages. The numbers of inner-cell-mass and trophoblast cells did not differ between parthenogenetic and fertilized embryos at the early blastocyst stage, but were significantly fewer in parthenogenetic embryos (11.1 and 32.7) than in fertilized embryos (14.9 and 52.0) at the expanded blastocyst stage. The numbers of dead inner-cell-mass and trophoblast cells were significantly more in parthenogenetic embryos than in fertilized ones at the early and expanded blastocyst stages. At the early blastocyst stage, although no difference was observed between parthenogenetic and fertilized embryos in the rate of division of trophoblast cells, the rate of division of inner-cell-mass cells was significantly lower in parthenogenetic embryos (4.4%) than in fertilized ones (9.7%). There was no difference in the rate of cell division between parthenogenetic and fertilized embryos at the expanded blastocyst stage. From these results, it was inferred that the number of cells in diploid parthenogenetic embryos does not differ from fertilized embryos until blastocyst formation, but becomes fewer than in fertilized embryos because more cells are dying in parthenogenetic embryos during the expansion of blastocysts. A low rate of cell division in the early blastocyst stage is thought to be one reason for the fewer number of inner-cell-mass cells in parthenogenetic embryos at the expanded stage, in addition to the presence of a large number of dead cells.