Biology · Developmental Biology

A 'morphogen' determines cell fate in a concentration-dependent manner. Below are statements from an experiment studying the gradient-dependent effect of the morphogen activin on cell fate by placing activin (4 nM)-secreting beads on unspecified cells from an early Xenopus embryo: A. Beads without activin did not elicit expression of either Xbra or goosecoid genes. B. Cells nearest to the beads receiving the highest concentration of activin induced the goosecoid gene, whose product specifies the frog's dorsal-most structures. C. Cells nearest to the beads receiving the highest concentration of activin induced the Xbra gene, whose product specifies the frog's dorsal-most structures. D. Cells farthest from the beads receiving negligible activin activate the Xbra gene and become blood vessels and heart. E. Cells farthest from the beads receiving negligible activin activated neither Xbra nor goosecoid, and the 'default' gene expression instructed cells to become blood vessels and heart. Which of the above observations and conclusions drawn are correct?