[Preimplantation development of monozygotic twins. Experiments on mouse embryos]

2-celled mouse embryos F1 (CBA X C57B1) and ICR were divided to separate blastomeres. The sister blastomeres were cultivated for 3 days in microdrops of synthetic medium under the layer of mineral oil. The development of twin embryos attained the late blastocyst stage. Vital observations of the cleavage of sister blastomeres were carried out, and the cells of twin embryos were calculated on slides. Sometimes the twin embryos contained the same number of cells but, as a rule, the number of cells did not coincide, i. e. the descendants of sister blastomeres differed by the cleavage rate. Possible causes of this phenomenon are discussed.