The primitive streak of the chick embryo develops from one of the two layers of cells of the stage XIII blastoderm, the epiblast. The other layer of cells, the hypoblast, seems to be necessary for the induction of the primitive streak and also determines its orientation--rotation of the hypoblast by 90 degrees is followed by a similar rotation of the embryonic axis. After stage XIII, the hypoblast is replaced by the invaginating endoderm and plays no further part in the development of the embryo. By means of a technique for disaggregation and reconstituting cells of stage XIII hypoblasts, we have been able to show that the two functions, induction and orientation are independent and that with reconstituted hypoblasts, the orientation of the primitive streak is determined by the epiblast.