The neural inductive signal is transferred to ectoblast in 1-2 h but a continued contact with mesoblast for 2-3 h is essential for neuralization in the chick area pellucida

In the area pellucida of the chick gastrula, the Hensen's node (HN) graft m ust contact the competent ectoblast for at least 4 h to promote neural indu ction. When we removed the grafted HN after 1 to 3 h and replaced it by a n on-inducing post nodal (PN) fragment, a 1-2 h contact with HN was found to be sufficient to promote neural induction. When HN graft was removed after 3 or 4 h and replaced by PN, the neural inductive response was substantiall y improved towards formation of archencephalic structures. Thus, our result s indicate that neural induction takes place in two steps. In the first ste p, a contact with HN for 1-2 h is sufficient to transfer the inductive sign al which is stabilized through a second step involving continued cell-cell contact with even non-inducing PN mesoblast.