The proliferative state, graft-site and contact-time of competent chick ectoblast determine the quality and quantity of neural induction by Hensen's node.

We have assessed the type and amount of neural tissue induced in the chick gastrula ectoblast with increasing duration of contact with the Hensen node inducer. At least 4 h contact is necessary to induce the ectoblast in the area pellucida, 9 h in the area opaca and even longer at the margin of overgrowth. In the area pellucida, the inductive response shifts from archencephalic type at 4-6 h contact to deuterencephalic type after 9 h contact. The induced tissue volume and cell number increased as graft-host contact increased from 4-9 h, and then decreased with longer contact. We suggest that in addition to the period of contact and the grafting position on competent ectoblast, the rate of cell proliferation may control the axial specificity and morphological organization of the induced neural tissue.