http://upload.wikimedia.org/wikipedia/commons/6/6d/Gray1109.png

The ventral part of the cloaca after its separation from the rectum, giving rise to the lower part of the bladder in both sexes, to the prostatic portion of the male urethra, and to the urethra and vestibule in the female. [TFD][VHOG]

urogenital sinus

uberon

In mammals the lowly monotremes still have a cloaca. Higher types have done away with this structure and have a separate anal outlet for the rectum. The monotreme cloaca shows the initiation of this subdivision. The cloaca has such includes only the distal part, roughly comparable to the proctodeum. The more proximal part is divided into (1) a large dorsal passage into which the intestine opens, the coprodeum, and (2) a ventral portion, the urodeum with which the bladder connects. (...) the development of the placental mammals recapitulates in many respects the phylogenetic story. In the sexually indifferent stage of placental mammal there is a cloaca. While the indifferent stage still persists, a septum develops, and extends out to the closing membrane. This divides the cloaca into two chambers: a coprodeum continuous with the gut above, and a urodeum or urogenital sinus below.[well established][VHOG]

UBERON:0000164

the term 'urogenital sinus' may refer to the primitive urogenital sinus present as a transient developmental structure in most mammals or it may refer to a condition in which an unseptated cloaca persists in animals longer than normal

• urogenital sinus lumen

• urogenital sinus mesenchyme

• urinary bladder
• urethra
• vagina

• urogenital sinus epithelium
• urogenital sinus lumen