Individual blastomeres of 2-, 4-, 8- and 16-cell embryos were fused with enucleated blastomeres of 2-cell embryos. The percentage of fused nuclear donor/recipient pairs was high (92 to 100%). Most of the reconstituted embryos cleaved twice within 24 h of in vitro culture and 83% to 95% developed to the morula or blastocyst stage. Increasing the number of blastomeres in reconstituted embryos by the construction of aggregates from blastomeres obtained 24 h after nuclear donor/recipient fusion was effective. All constructed aggregates reached the morula or blastocyst stage within 30 h of in vitro culture. The normal day-13 implantations were recorded after the transfer of reconstituted "half" embryos and aggregates when 8-cell blastomeres were used as nuclear donors.

Institute of Molecular Genetics Czechoslovak Academy of Sciences Praha