In avian gastrulation, Koller's sickle is a local thickening of cells that acts as a margin separating sheets of cells from posterior margin of avian blastoderms from hypoblasts as they migrate anteriorly to push primary hypoblast cells anteriorly to form a secondary hypoblast known as the endoblast.

The Posterior Marginal Zone (PMZ) of Koller's sickle can induce a primitive streak and Hensen's node, acting as an organizer.^[1]

References [link]

1. ^ Bachvarova RF, Skromne I, Stern CD. Induction of primitive streak and Hensen's node by the posterior marginal zone in the early chick embryo. Development. 1998 Sep;125(17):3521-34.

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