ABSTRACT
Orientation in the avian blastodisk is triggered by the force of gravity. Its manifestation is a gradual process of cell shedding from the lower layer, leading to the formation of an area pellucida. The marginal zone is the dominant source of a population of hypoblastic cells, which are responsible for the induction of the primitive streak in the epiblast. Posterior marginal zone cells have been demonstrated to move anteriorly into the forming hypoblast, while lateral marginal zone cells show a very limited movement. The marginal zone belt functions as a coordinating system, in which there is a very subtle equilibrium of inductive and inhibitory effects, which ensures the induction of a single primitive streak. The posterior marginal zone cells seem to gain their inductivity only after leaving the marginal zone belt and crossing Koller’s sickle into the hypoblast.
