Experimental embryology has shown that the ground state for the mesoderm is ventral, and special signals from the dorsal vegetal region (Nieuwkoop center) and then the dorsal mesoderm (Spemann organizer) cause dorsal development to occur (1). However, now that the molecular mechanisms for mesoderm formation are being elucidated, it is becoming clearer that various different growth factors contribute to both dorsal and ventral pattern (2). The most recent additions to our understanding come from dominant-negative mutations in bone morphoge-netic protein (BMP) receptors (3-5). These experiments show that the ventral mesoderm requires an active signal to form the most ventral tissues, such as red blood cells. When mutant BMP receptors are overexpressed and inhibit the normal BMP signal, the tissue develops into muscle, a more dorsal fate (and occasion-ally the most dorsal fate of notochord). The simplest conclusion is that BMPs act on the ventral mesoderm in normal development to induce the most ventral tissues, blood and mesenchyme.