Time-course of markers during neural induction and their regulation by signals.

Temporal hierarchy of deployment of 13 early neural markers. The colored lines at the bottom of the figure represent the period of expression of these genes, in relation to the time at which they are induced following a graft of Hensen's node into the area opaca (in hours on the scale above) and in relation to the stage of normal embryos at which they are expressed (stage shown above the time line). The diagrams above these stages schematize the domains of expression. The genes fall into three “epochs”: those induced by a node within 3 hours start to be expressed in normal embryos before streak formation (red). Those induced by a node in 4–5 hours begin their expression at the mid- to late-primitive streak stage (blue) and those that are induced by a node at 12–13 hours do not begin their expression until the end of gastrulation, in the forming neural plate (green).