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In the present study we placed germ discs (or fragments containing the deep central part of it) from unfertilized laid or extracted quail eggs on the deep side of the upper layer of isolated anti-sickle regions from unincubated chicken blastoderms. After culture in vitro of associations where the central deep part of the germ discs was in contact with the deep side of the upper layer (UL), we observed in about 30% of the cases the onset of embryonic development. Both associated parts play a role in the final formation of an embryo. Our experimental results, suggest that the delta ooplasm of the nucleus of Pander influences the cranial upper layer to segregate an endophyll layer. The definitive embryonic structures i.e. mesoderm, epiblast and neural plate are derived from the chicken upper layer by respectively normal gastrulation and (pre)neurulation phenomena. Our experiments seem to have some homology with the association experiments of isolated cortices from various regions of unfertilized Xenopus eggs implanted into the ventroequatorial core of a recipient 8-cell Xenopus embryo.

By placing a central subgerminal ooplasmic mass over isolated parts (alone or in association) of unincubated avian blastoderms and culture, we obtained an improvement in, or in some cases restoration of normal development. The evolution of small rectangular fragments (isolates) excised from different regions of the unincubated blastoderm was observed in association or not with subgerminal ooplasm. The only type of differentiation that was clearly distinguished in these isolates (taken from the caudocentral area centralis region) was a so-called 'primary neurula' formed by the endophyll and an associated thickened upper layer. In the present study, we also demonstrate that after removal of the area centralis from an unincubated caudal blastoderm quadrant, the upper layer (UL) and endophyll can no longer be restored from the associated subgerminal ooplasm (and form a miniature embryo), as is the case after removal of the endophyll alone. A deep layer (containing the endophyll) reformed during the migration of Rauber's sickle-derived cells into the neighbouring central subgerminal ooplasm only in the presence of the upper layer. This suggests that the upper layer has an early influence on the cells containing the original central deep ooplasm (delta ooplasm) to form the endophyll. The present study offers supplementary arguments in favour of the hypothesis that the endophyll is an inductor of preneurulation.

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.