Practice question

Gastrulation begins posteriorly because posterior marginal zone expresses highest Vg1/GDF1 and Wnt8c, cooperating with Koller's sickle to activate Nodal, Brachyury and Mixl1 in overlying epiblast at posterior edge of area pellucida. Epiblast cells undergo convergence toward midline and ingress at this posterior site, extending streak anteriorly as more cells join. Anterior marginal zone secretes Cerberus and DKK1 antagonizing Wnt and Nodal, preventing ectopic streak. Thus first morphological streak always appears at posterior end, progressively elongating toward presumptive head, defining AP polarity and tail-to-head growth.