Fig. 1. Organization of the pre-implantation and gastrulation-stage mouse embryo. (Left) An E3.5 blastocyst. The inner cell mass (ICM) gives rise to the epiblast lineages; the primitive endoderm (PrE) gives rise to parietal and visceral endoderms; the trophectoderm (TE) to the extra-embryonic ectoderm. (Right) An E6.5 gastrulation stage embryo with the organization of embryonic/extra-embryonic tissue layers in detail.