Fig. 4. Excessive growth of axial mesoderm and bifurcation of the embryonic axis in 9-9.5 dpc Mixl1^–/– embryos. (A) Dorsolateral view of a mutant embryo showing Brachyury (T)-expressing tissues in the bulge (arrow) formed at the caudal end of the embryonic axis and (B) ventral view of the same embryo showing the protrusion derived from the axial tissue mass (arrow). (C-E) Histological sections at the planes indicated in B show (C) T expression in the flat notochord-like structure, (D) the protruding tissues on the ventral side of the embryo (arrow indicates the same structure shown in B) and (E) the bulge that extends dorsally through a split in the neural plate. hf, head fold; al, allantois; nc, notochord-like structure; ne, neuroectoderm; nf, neural fold.