The effects of antero-posterior reversal of lengths of the primitive streak in the chick

Abstract In chick blastoderms at primitive streak stage, lengths of the primitive streak were cut out and replaced with their antero-posterior orientation reversed. In some experiments the region immediately in front of the primitive streak (presumptive prechordal head) was also included in the excisedpiece. Control operations involving excision and replacement without reversal were also performed. The embryos were subsequently grown in vitro by Waddington’s technique. After reversal of a variety of different parts of the streak at various developmental stages, many cases of regulative development were obtained. In these, the original orientation of the blastoderm was maintained, and while there were abnormalities of various kinds in the embryos, they were no different from the abnormalities found in the controls. Very occasionally the regulated axis was partially doubled after a reversal, though not after a control operation. A few specimens which had undergone reversal of long pieces of the primitive streak and had completely healed showed a failure of regulation in that there was some tendency for the reversed-piece to develop according to its own orientation. But at best this reversed differentiation was very distorted and incomplete. Evidently the orientation of the primitive streak does not at any stage control the orientation of the embryo; and the primitive streak, when it is fully developed and contains most of the presumptive axial material, is highly labile in its powers of differentiation. In spite of its well-known ‘organizer' activity, the primitive streak is subject to control by the surrounding blastoderm.