To learn the origin and early migration of notochordal cells in the chick embryo, tritiated thymidine-labeled grafts were implanted into homologous positions in Hensen's node of recipient embryos at homologous stages of development. The host embryos were reincubated to Hamburger-Hamilton stages 5-9, fixed, embedded in paraffin, and serially sectioned. Sections were coated with photographic emulsion and exposed for 7-30 days. Labeled cells were found in the notochord or chordamesoderm in 31 of the 35 embryos. Transplants to the dorsal portion of the node always formed notochord. Transplants to the ventral node formed notochord only when implanted during or after the definitive streak stage (H-H stage 4+), indicating that the chorda center is located in the dorsal layer. Moreover, the results suggest that the node depletes itself in producing the notochord: labeled cells migrated into notochord or chordamesoderm anterior to the primitive pit, rather than remaining in the node or primitive pit. Because transplants to the anterolateral portion of the node contributed only to trunk notochord and several transplants to the posteromedial portion contributed only to head notochord, Hensen's node may be equivalent to the median lip of a blastopore, over which prenotochordal material in the dorsal (epiblast) layer passes during gastrulation.