SPATIAL RELATIONSHIP BETWEEN ENDOPHYLL, PRIMORDIAL GERM-CELLS, SICKLEENDOBLAST AND UPPER LAYER IN CULTURED AVIAN BLASTODERMS

By isotopic quail-chicken chimera experiments of the Rauber's sickle o r by radioactive labelling and isotopically replacing of the caudal en dophyllic sheet in unincubated avian blastoderms, followed by culture, we demonstrated that the displacement of the endophyll by the cranial ly extending sickle endoblast is not exclusively a mechanical phenomen on, as suggested by earlier studies (Vakaet, 1962 a, b). Indeed, our s tudy suggests that the sickle endoblast also migrates centripetally ve ry soon (already after 5 h) in and through the caudal endophyll before ingression of upper layer cells takes place. We also describe the ear ly spatial relationship between the three elementary tissues (endophyl l, Rauber's sickle, upper layer) (Callebaut et al, 1996a) and the indu ction phenomena between quail sickle endoblast and chicken upper layer (UL) during the formation of the primitive groove. The latter already develops before ingression occurs. We found no evidence for an endoph yllic origin of avian primordial germ cells.