“…First, polarity and, therefore, pattern appears to already exist in the epiblastal ectoderm before gastrulation as evinced by asymmetric patterns of gene expression typically encountered within epiblast (e.g., Bmp4,Otx2,Fgf8,T [Brachury],Cripto,Wnt3,Nodal,Hesx1,Hex,, VE-1, Lim1 [Lhx1], GATA, Dlx, and Dkk1; Lawson et al, 1991;Lawson and Pedersen, 1992;Dono et al, 1993;Simeone et al, 1993;Ang and Rossant, 1994;Crossley and Martin, 1995;Belo et al, 1997;Varlet et al, 1997;Eyal-Giladi, 1997;Beddington and Robertson, 1998;Biben et al, 1998;Ding et al, 1998;Shawlot et al, 1998;Thomas et al, 1998;Liu et al, 1999;Pera et al, 1999;Crossley et al, 2001;Tian et al, 2002;Chapman et al, 2002). This asymmetry is mirrored by the underlying hypoblast (visceral endoderm;e.g., Otx2, Lim1, Hesx1, Hex, HNF3, Lefty1, Nodal (Acampora et al, 1995;Thomas and Beddington, 1996;Belo et al, 1997;Filosa et al, 1997;Varlet et al, 1997;Thomas et al, 1998;Meno et al, 1999;Shimono and Berhinger, 1999;Kim...…”