Abstract
The replication behavior of the X chromosome was examined in diploid 8.5- and 9.5-day parthenogenetic LT/Sv mouse embryos showing growth retardation and morphological anomalies. A single late replicating X chromosome was found in the extraembryonic and in the embryonic regions. The frequency of cells with a late replicating X was significantly lower in the extraembryonic region of the parthenotes than in that of normally fertilized embryos.