Implantantions of the transverse neural fold with or without oral entoderm were operated in blastocoele. If the neural fold is implanted with the oral entoderm (or oral ecto-entoderm) a complete supernumerary mouth is obtained. This result can be explained admitting that the neural fold induces the anterior, undetermined entoderm to become oral entoderm: this, on the other hand, acting on the above situated ectoderm, induces it to form oral structures. If only oral ecto-entoderm is implanted in blastocoele no supernumerary mouths or oral structures are formed. The same negative result is obtained when the transverse neural fold is implanted alone.