Interaction of epiblast and hypoblast in the formation of the primitive streak and the embryonic axis in chick, as revealed by hypoblast-rotation experiments

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Abstract
Three types of experiments were performed to determine the interaction between the epiblast and hypoblast for primitive streak formation: (1) Hypoblasts of blastoderms from stages XIII E.G & K to 3 H & H were separated from the epiblasts and rotated by 90° counterclockwise; (2) hypoblasts from stages XIII E.G & K to 3 H & H blastoderms were rotated by 180°; (3) hypoblasts were exchanged between blastoderms of different developmental stages and placed at 90° counterclockwise to the axis of the recipient epiblast. In all blastoderms studied only a single PS developed. After rotation of the hypoblast by 90°, the direction of the PS was according to the orientation of the hypoblast at stage XIII, whereas at older stages it gradually shifted towards the axis of the epiblast. At stage 3− H & H the PS is already imprinted in the epiblast and cannot be shifted. After rotation of the hypoblast by 180° the PS originated at the point near the marginal zone at which the inductive part of the hypoblast interacted with a competent epiblast. Conclusions are drawn about the dynamics of the inductiveness of the hypoblast and the competence of the epiblast for the PS formation and orientation.