External genitalia

Indifferent period

The cloacal membrane blocks off the cloaca from the outside. At week 6 (S18), the mesoderm around this membrane expands to form three protuberances: ventrally the unpaired genital tubercle (tuberculum genitale), and laterally the genital swellings (tubercula labioscrotalia). The primary urethral folds (plicae urogenitales) develop medially to the genital swellings. The cloacal membrane is thus found in a depression known as the external cloaca. The internal cloaca is subdivided by the urorectal septum into the primary urogenital sinus and the anorectal canal. The body of the perineum forms where the urorectal septum meets the cloacal membrane. Ventral to the perineum, the cloacal membrane becomes the urogenital membrane and forms the floor of the primary urethral groove. Dorsal to the perineum, it becomes the anal membrane and forms the anal pit.
The floor of the urethral groove develops from the ectoderm of the cloacal membrane. The urethral plate originates from the endoderm of the ventral part of the cloacal membrane as a median septum, which grows into the primordial phallus (extended genital tubercle). At around S18-19, the urogenital membrane disintegrates. Due to this, the phallic part of the urogenital sinus becomes open to the outside.