Closure of the embryonic bladder based on the results of the study of 25 human embryos (Crown-Rump length: 5mm-60mm) and 8 Born's reconstructions, is presented. It is a complex but well regulated stage of organogenesis. Division of the cloaca depends on caudal regression, development of the genital tubercle and growth of the urorectal fold which joins the cloacal membrane. The didermal cloacal membrane (composed of the ecto and endodermal layers) opens on its caudal part. On the cranial part, there is a cloacal plug, composed of packed cells, which strengthens the inferior part of the genital tubercle. After progressive vacuolation, this plug is colonized by mesodermal tissue derived from the urorectal fold, which assumes the development of the anterior perineum. This stage depends on cranial growth of the genital tubercle. On a human embryo (31 mm Crown-Rump length, 52 days, 10 Weeks of amenorrhea), the bladder is closed, urachus obturated and urethra patent. The vesico-ureteral junction, and prostatic utricle are present and the colliculus seminalis is well developed. A temporary obstruction (caused by asynchronism between urachus closure and urethral opening) can induce antenatal vesicoureteral reflux which could regress after birth. However if urethral obstruction is definitive (posterior urethral valves or atresia), reflux persists. Antenatal sonographic bladder identification is possible after 10 weeks of amenorrhea with visualisation of the two umbilical arteries by color duplex ultrasound. If the urachus remains patent, it can be visualized inside the arterial umbilical triangle, on a transverse view.