Fig. 5. Cellular distribution of Brachyury expression as seen in sagittal (A,B) and horizontal (C,D) semi-thin sections from specimens in Fig. 4. (A,B) At stage 2, Brachyury transcripts are confined to epiblast cells (e) in the posterior third of the embryonic disc. Anterior (A) and posterior (B) margins (epiblast-trophoblast junction) are marked with single and double asterisks, respectively. (C,D) No Brachyury expression in epiblast or mesoderm of the anterior half of the primitive streak at stage 4 (arrowhead marks primitive groove in C) but strong Brachyury expression in the posterior half of the primitive streak (D) in epiblast (e) and ingressing mesoderm (i). Scale bar: 40 µm.