Practice question

Eyal-Giladi and Kochav staging scheme describes chick prestreak stages X to XIV. Hypoblast islands start appearing stage X-XI, spreading anteriorly, continuous layer completed by stage XIII just before appearance of primitive streak stage XIV. Before stage XIII hypoblast incomplete, after completion it covers entire subgerminal cavity and secretes Dkk1 and Cerberus inhibiting streak anteriorly, restricting streak to posterior. Therefore hypoblast formation completion at stage XIII marks transition from area pellucida to gastrulation competence and prerequisite for primitive streak initiation. This illustrates conserved developmental logic of morphogen gradients patterning embryonic axes through Wnt and BMP antagonism.