The longitudinal duplication of the cloacal membrane, which is influenced by specific environmental or genetic factors in embryonic development, leads to an increase in the mesoderm on its cephalic side to form two germinal nodules, each developing into a penis, or a fusion defect to form a branching penis when the germinal nodules are extended to form a penis. If the embryonic defect occurs at the head of the cloacal membrane, only the head of the penis can be duplicated, and if the defect occurs on the posterior and caudal side of the young embryo, it is often accompanied by lumbosacral spinal deformities, anal atresia, scrotal splitting and duplication of the bladder, in addition to a complete duplication of the penis.