Two-cell embryos from ROSA26 mice were cultured in the presence of thymidine or methyl-[³H]thymidine for 65 hours. The number within the parentheses indicates the percentage of embryos either developed to blastocysts or degenerated. The absence of the ICM cells from these trophoblast vesicles was confirmed by microscopic examination of serial sections after placing them inside an oviduct that functioned as a cassette (results are shown in Fig. 6). These vesicles were viable since 90% of them escaped from their zona pellucida in vitro after 48 hours of culture (data not shown).