noun
: a distensible membranous sac in many vertebrates that serves for the temporary retention of the urine, that in man is situated in the pelvis in front of the rectum, receives the urine from the two ureters and discharges it at intervals into the urethra through an orifice closed by a sphincter, is lined with transitional hypoblastic epithelium, and develops from the proximal part of the allantois of the embryo, that in the lowest mammals and in birds, reptiles, and amphibians opens separately into the cloaca, and that in fishes if present is not homologous with that of the higher vertebrates but is a dilatation of a ureter or of the united ureters