Only the endophyll-Rauber's sickle complex and not cells derived from the caudal marginal zone induce a primitive streak in the upper layer of avian blastoderms.

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Abstract
From unincubated quail blastoderms, we have excised, caudal marginal zones (caudally from Rauber's sickle), upper layer fragments covering Rauber's sickle or Rauber's sickle fragments alone (as controls), and placed 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 isolated central upper layer discs of prestreak chicken blastoderms from which the deep layer was previously removed. Only the Rauber's sickle-derived cells (sickle endoblast cells), placed on unincubated or shortly incubated blastoderms induce, after culture, a primitive streak (PS) and a normal embryo. This indicates, together with previous experimental evidence, that even in the presence of endophyll, neither the deep part of the caudal marginal zone nor the upper layer above it can induce a primitive streak. This experimental study affords further evidence that the function of the avian Rauber's sickle is homologous to the function (mesoderm induction) of the vegetal dorsalizing cells (Nieuwkoop centre) in amphibian blastulas.

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