The thin bilaminar membrane derived from the prechordal plate that is devoid of mesoderm and formed by the apposition of the stomodeal ectoderm with the foregut endoderm; after the embryonic head fold has evolved it lies at the caudal limit of the stomodeum, forming a septum between the primitive mouth and pharynx; the membrane eventually disappears, and thus a communication is established between the mouth and the future pharynx. [ ]