Practice question

Time-lapse imaging of chick gastrulation shows primitive streak first becomes morphologically visible at posterior marginal zone near Koller's sickle as cells accumulate expressing Brachyury epiblast marker. Posterior site coincides with Vg1 and Wnt8c expression domain inducing streak, then extension proceeds anteriorly driven by convergent extension and planar cell polarity. Appearance at anterior would invert polarity and misposition node. Normal development therefore initiates streak posteriorly, with Hensen's node forming later at anterior tip. Posterior origin ensures caudal to cranial development sequence and proper axis orientation. This illustrates conserved developmental logic of morphogen gradients patterning embryonic axes through Wnt and BMP antagonism.