In the present report we describe the role of the three elementary tissues during early embryonic patterning. Not the absolute quantity, but the local spatial relationship, boundaries and contact zones of these elementary tissues play a determining role at the start of incubation in vitro. The formation of (hemi) primitive streaks or parallel asymmetric unequal hemi-primitive streaks in different hemi-sectioned (mediosagittal, oblique, transverse) or not sectioned avian blastoderms were compared and explained by a similar mechanism of strong sliding of upper layer cells in the concavity of the Rauber’s sickle (RS). This occurs by induction on the upper layer of the Rauber’s sickle material (positional information), without interference of the caudal marginal zone (in which the area vasculosa develops). There exists a caudo-centrally directed (from sickle-horn top to the central median part) induction polarity along side the internal border of the RS, explaining the so-called “polonaise movement” of upper layer cells. The caudo-centrally directed sliding of upper layer (UL) cells (which will transform in a half-PS and half-mesoblast mantle) in the concavity of Rauber’s sickle (RS) at different levels is an extensive strong mechanical phenomenon. So, when unimpaired, these UL cells can cross the midline region. This indicates that in normally developing blastoderms a mechanical left-right equilibrium exists. When no RS-material is present, no sliding of UL cells in the neighboring area centralis occurs. There exists an obvious homology in avian blastoderms with the latero-lateral or caudo-cranial mosaic phenomena observed in other classes of chordates (Ascidia and Amphibia). Gastrulation (HOXB1 expressing domain) and neurulation (OTX2 expressing domain) can be explained by the presence of two different organ-group-organizing ooplasms: respectively у−ooplasm or δ−ooplasm−containing structures (Rauber’s sickle or endophyll).