Fig. 1. Mouse embryos at the periimplantation (A, day 4.5) and post-implantation (B, day 6.0; C, day 7.0) stages of development, showing the allocation of derivatives of the inner cell mass/epiblast, primitive endoderm and trophectoderm, to different tissue compartments of the conceptus. The inner cell mass is the precursor of the epiblast of day 6.0 embryos, and the transition between these two tissues is likely to be a progressive process (Rathjen et al., 2002). The epiblast gives rise to ectoderm, mesoderm and definitive endoderm. The primitive endoderm of the day 4.5 embryo is derived from the inner cell mass. It differentiates into parietal and visceral endoderm (both contribute to fetal extra-embryonic membranes). The trophectoderm gives rise to the ectoplacental cone, extra-embryonic ectoderm and the trophoblast giant cells. The Reichert's membrane is a composite layer of trophoblast giant cells, the basement membrane and the parietal endoderm.