Charaterization of the TCF/Lef-LacZ transgene expression at gastrulation. Lateral (A) and posterior (A') views of an early streak embryo stained for ß-galactosidase activity. Transverse sections (A1, A2) show LacZ staining at the posterior part of the embryo marking streak initiation. Lateral and posterior views of a mid-streak (B, B') and late streak (C, C') embryos and transverse sections through them (B1, B2, C1, C2 respectively) showing LacZ staining in the primitive streak and wings of mesoderm. Lateral (D) and posterior (D') views of a late bud embryo positive for the transgene and sections through it (D1, D2) along the indicated planes. ES, early streak; LB, late bud; LS, late streak; MS, mid-streak.