In avian

Bird

Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...

 gastrulation

Gastrulation

Gastrulation is a phase early in the embryonic development of most animals, during which the single-layered blastula is reorganized into a trilaminar structure known as the gastrula. These three germ layers are known as the ectoderm, mesoderm, and endoderm.Gastrulation takes place after cleavage...

, Koller's sickle is a local thickening of cells that acts as a margin separating sheets of cells from posterior margin of avian blastoderm

Blastoderm

-In amniotes:A blastoderm is the layer of cells formed at one pole of macrolecithal eggs such as the yolky egg of birds. The yolk prevents the division from taking place through the egg, resulting in meroblastic cleavage during the many cleavage divisions...

s from hypoblast

Hypoblast

The hypoblast is a tissue type that forms from the inner cell mass. It lies beneath the epiblast and consists of small cuboidal cells.Extraembryonic endoderm is derived from hypoblast.-External links:...

s 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

Primitive streak

The primitive streak is a structure that forms during the early stages of avian, reptilian and mammalian embryonic development.-Introduction:...

 and Hensen's node

Hensen's node

The primitive knot is the organizer for gastrulation in vertebrates.-Diversity:* In birds it is known as "Hensen's node", and is named after its discoverer Victor Hensen....

, acting as an organizer.