Figure 5

Time-dependent specification of mesodermal tissues by Nodal signals. Embryos were treated with SB-431542 at different stages of development between the mid-blastula transition and the onset of gastrulation, and were examined morphologically at 14, 24 and 48 h for visible trunk somites, notochord, blood, Kupffer's vesicle, hatching gland, a beating heart and a functional circulatory system. The steep curves for somites, hatching gland, Kupffer's vesicle, and beating hearts indicates that specification of these tissues occurs rapidly and in a stepwise fashion. Specification of notochord and blood occurs over a longer period. The number of embryos examined at each time point is indicated.