A proposed model for the activity of Notochord (No)-derived Sonic hedgehog (Shh). The No secretes Shh that acts on the ventral neural tube (NT) and also traverses the sclerotome (Scl) which is both a pathway for ligand movement and also a target of its activity. We propose that a ventral to dorsal gradient of ligand is created in Scl and plays a pivotal role both on myotome as well as on motoneuron development. Shh is thus presented to the target epithelial cells via their basal domain.