Visualizing a morphogen gradient: phosphorylated Smadi forms a gradient, maximal in the ventral, in the Xenopus(爪蟾) gastrula(原肠胚 Dorsal Ventral Dorsal Ⅴ entral Blastopore Transverse section at the Side view level of white arrowsDorsal Ventral Blastopore Side view Transverse section at the level of white arrows Visualizing a morphogen gradient: phosphorylated Smad1 forms a gradient, maximal in the ventral, in the Xenopus（爪蟾） gastrula（原肠胚） Fig. 7a