Myocardial specification, but not heart tube morphogenesis, is normal in the absence of FNs. Whole-mount immunohistochemistry with antibody MF-20 to mark myosin-positive cells in wild-type E 8.0 (A), FN.null-/- E 8.0 (B), FN.null-/- more severe class at E 8.5 (C and D), wild-type E 8.5 (E), and FN.null-/- less severe class at E 8.5 (F) mouse embryos. Note that myosin-positive cells are situated normally within the mutant embryos, even when no primitive heart is formed. In the mutant embryos in which a heart is formed, myosin-positive cells consistently display abnormal heart morphogenesis. HP, heart primordium; NT, neural tube; HF, head fold; Al, allantois.