The inducing capacities of the primary hypoblast as revealed by transfilter induction studies.

1. The inducing powers of the primary hypoblast of the chick embryo were studied by the insertion of a TH millipore filter between the hypoblast and the reacting epiblast. 2. Two successive inducing capacities were discovered in the hypoblast. The first to appear and disappear is the inductor of the primitive streak, while the second is a prosencephalic inductor. 3. The formation of a mature primitive streak which contains a Hensen's node is dependent on direct cellular contact between the epiblast and hypoblast, and most probably on the inclusion of hypoblastic cells in the forming node and notochord. 4. The formation of spinal cord-like structures and undifferentiated neural plates can be related to the presence of nodeless primitive streaks in the blastoderms. These neural structures are probably induced by the mesoderm produced by the defective primitive streaks, which were unable to form notochord or somites. 5. The primary hypoblast, by virtue of its prosencephalic inducing power, can be compared with the presumptive pharyngeal endoderm of the amphibian embryo.