ONLY THE ENDOPHYLL RAUBERS SICKLE COMPLEX AND NOT CELLS DERIVED FROM THE CAUDAL MARGINAL ZONE INDUCE A PRIMITIVE STREAK IN THE UPPER LAYER OF AVIAN BLASTODERMS

From unincubated quail blastoderms, we have excised, caudal marginal z ones (caudally from Rauber's sickle), upper layer fragments covering R auber's sickle or Rauber's sickle fragments alone (as controls), and p laced them on the ventral side of the cranial quadrant of unincubated chicken blastoderms. Also, quail Rauber's sickle fragments, all or not associated with quail endophyll, were placed on the ventral side of i solated central upper layer discs of prestreak chicken blastoderms fro m which the deep layer was previously removed. Only the Rauber's sickl e-derived cells (sickle endoblast cells), placed on unincubated or sho rtly incubated blastoderms induce, after culture, a primitive streak ( PS) and a normal embryo. This indicates, together with previous experi mental evidence, that even in the presence of endophyll, neither the d eep part of the caudal marginal zone nor the upper layer above it can induce a primitive streak. This experimental study affords further evi dence that the function of the avian Rauber's sickle is homologous to the function (mesoderm induction) of the vegetal dorsalizing cells (Ni euwkoop centre) in amphibian blastulas. (C) Inra/Elsevier, Paris.