urogenital sinus n the ventral part of the embryonic mammalian cloaca that is formed by the growth of a fold dividing the cloaca where the gut and allantois meet and that eventually forms the neck of the bladder and some of the more distal portions of the genitourinary tract
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sinus urogenitalis an elongated sac formed by division of the cloaca in the early embryo, communicating with the mesonephric ducts and primordial urinary bladder; it forms most of the bladder in both sexes, the vestibule, urethra, and vagina in the female, and most of the urethra in the male.Medical dictionary. 2011.