Fig. 3. Spemann-Mangold organizer secreted factors antagonize ventral signals provided by BMPs.

BMPs are secreted by a wide region at the ventral side of the embryo and are antagonized by organizer secreted factors such as Chordin, Noggin and Follistatin (blue oval). These factors directly bind to BMPs in the extracellular space of ectoderm, mesoderm and endoderm and thereby pattern these three germ layers.