The primitive streak forms a longitudinal midline structure in the narrower posterior (caudal) region of the developing embryo on its dorsal side.

[4] The primitive streak establishes bilateral symmetry, determines the site of gastrulation, and initiates germ layer formation.

[2] The primitive streak forms a longitudinal midline structure in the narrower caudal (posterior) region of the developing embryo on its dorsal side.

[13] Cells overlaying Koller's sickle in the posterior end of the chick embryo move towards the midline, meet and change direction towards the center of the epiblast.

[8][14] As these cells move and concentrate at the posterior end of the embryo, the streak undergoes a single- to multi-layered epithelial sheet transition that makes it a macroscopically visible structure.

[8] This region has a defined anterior-to-posterior gradient in its ability to induce the primitive streak, with the posterior end having the highest potential.

[20][21][22][23][19] Similarly, Vg1 (a transforming growth factor beta family member (TGF-β)) misexpression and grafts of the posterior marginal zone[16] in chicks can also induce ectopic streaks, but only within the marginal zone of the embryo,[22] indicating a specific characteristic of this region in its ability to induce streak formation.

[24] In addition, mutating the intracellular negative regulator of Wnt signaling, Axin,[25] and misexpression of the chick cWnt8C[26] produces multiple streaks in mouse embryos.

[19] Eventually, the hypoblast gets displaced anteriorly by the moving endoblast, allowing streak formation at the posterior end.

[19] Similarly to the hypoblast in chick, the AVE in the mouse secretes two antagonists of Nodal signaling, Cerberus-like, Cerl, and Lefty1.

[19][28] In mouse, Cer-/-; Lefty1-/- double mutants develop multiple streaks[17] as indicated by ectopic expression of Brachyury and can be partially rescued by the removal of one copy of the Nodal gene.

[17] In the mouse, the AVE restricts streak formation through the redundant functions of Cer1 and Lefty1, which negatively regulate Nodal signaling.

[17] The role of the mouse's AVE in ensuring the formation of a single primitive streak is evolutionarily conserved in the hypoblast of the chick.

[17][19] Another important pathway in modulating formation of the primitive streak is FGF, which is thought to work together with Nodal to regulate this process.