The neural inductive response of competent chick ectoblast decreases away from the host axis and correlates with an increased proliferative activity

We have assessed the quality and quantity of the neural inductive response of the chick gastrula ectoblast located at increasing distance from the hos t axis. In a stage 4 gastrula, entire ectoblast exhibits neural competence. The quality of induced neural tissue shifts from deuterencephalic type in the area pellucida to archencephalic type in the area opaca and primitive m edullary or palisade type at the margin of overgrowth with a concomitant re duction in the number of induced neural cells. In contrast, the mitotic and H-3-TdR labelling frequencies in the competent ectoblast increase with inc reasing distance from the host axis and in a proportion inverse to the amou nt of induced neural tissue. It is suggested that the strong neural inducti ve response is correlated with low proliferative activity, or longer cell c ycle time, of the competent ectoblast.