THE ROTATED HYPOBLAST OF THE CHICKEN-EMBRYO DOES NOT INITIATE AN ECTOPIC AXIS IN THE EPIBLAST

In the amniotes, two unique layers of cells, the epiblast and the hypo blast, constitute the embryo at the blastula stage. All the tissues of the adult will derive from the epiblast, whereas hypoblast cells will form extraembryonic yolk sac endoderm. During gastrulation, the endod erm and the mesoderm of the embryo arise from the primitive streak, wh ich is an epiblast structure through which cells enter the interior. P revious investigations by others have led to the conclusion that the a vian hypoblast, when rotated with regard to the epiblast, has inductiv e properties that can change the fate of competent cells in the epibla st to form an ectopic embryonic axis. Thus, it has been suggested that the hypoblast normally induces the epiblast to form a primitive strea k at a specific locus. In the work reported here, an attempt was made to reexamine the issue of induction. In contrast to previous reports, it was found that the rotated hypoblast of the chicken embryo does not initiate formation of an ectopic axis in the epiblast. The embryonic axis always initiates and develops according to the basic polarity of the epiblast layer. These results provoke a reinterpretation of the is sues of mesoderm induction and primitive streak initiation in the avia n embryo.