Yolk and ooplasmic labeling permitted us to bridge the gap between the radially symmetrical ooplasmic structures (α, β, γ, δ-ooplasms) and their fate during early embryonic development.  At the moment of bilateral symmetrization (characterized, long before the formation of the primitive streak, by the appearance of Rauber’s – Koller’s sickle), an oblique sickle-shaped uptake of γ-ooplasm (Rauber’s sickle material, containing Vg1) and δ-ooplasm (endophyll) into the deeper part of the avian blastoderm occurs. Contrary to common belief, the Nucleus of Pander (superficial part) plays an important role during early development (before laying) by giving rise to the deep δ-ooplasm-containing endophyllic layer (inducing preneurulation and directing gastrulation) and from which the primordial germ cells will segregate into the germinal crescent. Rauber’s sickle (containing Vg1) and not the caudal marginal zone is the early primary organizer of the avian blastoderm. It induces the Anlage fields in the upper layer cells localized within its concavity (intramural effect). Later, after formation of a primitive streak, Rauber’s sickle induces the formation of the coelomo-cardio-vascular system at its outer side in the caudal marginal zone (extramural effect). We found evidence that avian conjoined (Siamese) twins are formed by fusion (parapagus) and not by fission. The autochthonous Rauber’s sickle has an inhibitory effect (competitive inhibition) on exochthonous Rauber’s sickle material or sickle endoblast placed on the anti-sickle region of whole blastoderms. The spatial spreading of Rauber’s sickle material in relationship with the neighboring upper layer influences left/right asymmetry and determines regulation in ovo versus mosaic (hemi-primitive streak) development in vitro. The real significance of the caudal marginal zone versus Rauber’s sickle is discussed. The penetration of radioactively labeled egg white-derived material through the vitelline membrane and its massive selective accumulation into the epigerminal, the subgerminal space and perigerminal yolk region, is demonstrated.