Practice question

Initial breaking of radial symmetry in amphibians occurs at fertilization when sperm aster forms in vegetal cortex. Astral microtubules guide cortical rotation moving dorsal determinants 30 degrees away from sperm entry point. Sperm entry site therefore becomes ventral pole while opposite side accumulates Wnt11, Dishevelled and GBP, stabilizing beta-catenin dorsally. Gravity can bias but not determine axis; genetic differences and uterine orientation irrelevant in oviparous amphibians. Thus point of sperm entry sets dorso-ventral axis by directing cortical rotation direction and dorsal determinant transport. This illustrates conserved developmental logic of morphogen gradients patterning embryonic axes through Wnt and BMP antagonism.