The urinary bladder and urethra are not derived from the intermediate mesoderm but are endodermal derivatives. They develop from the urogeni-tal sinus, which is a derivative of cloaca (as already described, the cloaca is the dilated terminal part of the hind-gut). Partitioning of the cloaca by the urorectal septum into two parts, ie, urogenital sinus (the anterior part), and anorectal canal (the posterior part) has already been described in detail in chapter 6 and will not be repeated here. Students are advised to revise it to properly understand the normal development and abnormalities of urinary bladder and urethra.
The urogenital sinus is continuous superiorly with the allantois and is bounded inferiorly by the urogenital membrane (which is the anterior division of the cloacal membrane). The two mesonephric ducts open into the posterior wall of the urogenital sinus.
During the 5th week, the urogenital sinus can be divided into three parts: vesical part, pelvic part and phallic part. The vesical part (also called vesico-urethral canal), which is the upper expanded portion of the urogenital sinus gives rise to the urinary bladder in both sexes. Its terminal part also gives rise to most of the female urethra and the proximal half of the prostatic urethra in the male. The pelvic part, which is the middle narrower part of the sinus, forms the terminal part of female
urethra. In the male the pelvic part of the urogenital sinus forms the distal | half of the prostatic urethra and whole '< of the membranous urethra. The phallic part of the urogenital sinus is the lowermost part of the urogenital sinus and grows toward the genital tubercle (described later). This portion of the urogenital sinus gives rise to the penile urethra in the male and vestibule of vagina in the female.