 Shortly after the beginning of dorsoventral patterning, Shh is expressed along the basal plate (the bottom) of the embryo, which functions in "ventralization of the neural tube, promotion of growth and proliferation, and formation of the hypothalamus".As the embryo continues to develop, "Shh" expression characteristic of the ZLI extends dorsally to form a wedge that eventually narrows to a strip at approximately 22 somites (the number of developed rhombomeres) or less than one day in zebrafish.Although "Shh" expression extends dorsally from the basal plate, the ZLI is capable of forming even without the basal plate or mesodermal tissue.