Serves to facilitate the differentiation of either mesoderm or endoderm either as a cofactor in an instructive signal or by providing permissive environment. Homodimer. Abundant in ovaries and eggs, and equally distributed among all blastomeres. Concentrated in the streaming cytoplasm in just-fertilized eggs. Evenly partitioned during cleavage among all blastomeres. Absent in the yolk cell during cleavage, blastula and gastrula stages. Distributed homogeneously among all cells of the gastrula. Belongs to the TGF-beta family.