Serves to facilitate the differentiation of either mesoderm or endoderm either as a cofactor in an instructive signal or by providing permissive environment. Homodimer. Vegetal region of the egg. Abundant in oocytes and present throughout cleavage and gastrula stage. Not readily detected at a stage when somitogenesis is nearly complete in 24 hours embryos. Steady state level decreases in a continuous fashion with developmental age. Belongs to the TGF-beta family.