Low levels of BMPs allow dorsal cell fate specification (e.g., neural tissue; formation of head structures), whereas higher levels of BMPs direct
ventral cell fate specification (e.g., epidermis, blood, formation of tail structures).
Yet, in the absence of detailed investigations of expression profile and developmental
fate of mesodermal midline
cells in basal ecdysozoans and deuterostome ambulacrarians, the nature of
ventral midline tissue in urbilaterians remains undecided.