Further study of the urorectal septum in staged human embryos

Study was carried out on serial section of 45 human embryos of stages 13 to 23. The urorectal septum develops from three folds: single superior and two lateral ones. Development of the urorectal septum begins at stage 13 when the superior fold between the hindgut and allontois appears. At stage 15, all three folds unite to from complete septum, reaching the cloacal membrane and dividing the cloaca into anterior compartment, the primary urogenital sinus and the posterior compartment rectum.