A SPINAL-CORD FATE MAP IN THE AVIAN EMBRYO - WHILE REGRESSING, HENSENS NODE LAYS DOWN THE NOTOCHORD AND FLOOR PLATE THUS JOINING THE SPINAL-CORD LATERAL WALLS

The spinal cord of thoracic, lumbar and caudal levels is derived from a region designated as the sinus rhomboidalis in the 6-somite-stage em bryo, Using quail/chick grafts performed in ovo, we show the following , (1) The poor plate and notochord derive from a common population of cells, located in Hensen's node, which is equivalent to the chordoneur al hinge (CNH) as it was defined at the tail bud stage, (2) The latera l walls and the roof of the neural tube originate caudally and lateral ly to Hensen's node, during the regression of which the basal plate an lage is bisected by floor plate tissue, (3) Primary and secondary neur ulations involve similar morphogenetic movements but, in contrast to p rimary neurulation, extensive bilateral cell mixing is observed on the dorsal side of the region of secondary neurulation, (4) The posterior midline of the sinus rhomboidalis gives rise to semitic mesoderm and not to spinal cord, Moreover, mesodermal progenitors are spatially arr anged along the rest of the primitive streak, more caudal cells giving rise to more lateral embryonic structures, Together with the results reported in our study of tail bud development (Catala, M., Teillet, M. -A. and Le Douarin, N.M. (1995). Mech. Dev. 51, 51-65), these results show that the mechanisms that preside at axial elongation from the 6-s omite stage onwards are fundamentally similar during the complete proc ess of neurulation.