The caudal end of the hindgut is lined with endoderm and is known as the cloaca. At the end of the cloaca, the there is a cloacal membrane which separates the endoderm from the surface ectoderm.
As development proceeds, a wedge of mesenchyme migrates caudally to divide the cloaca into dorsal and ventral aspects. This wedge of mesenchyme is known as the uro-rectal septum and it marks the division between the urinary system and the genital system. On the ventral side, attaching to the allantois is the primitive uro-genital sinus. The primitive ano-rectal canal lies dorsally. As the division is completed, at approximately 7 weeks, the cloacal membrane is also divided into dorsal and ventral parts. The ventral portion forming the uro-genital membrane and the dorsal portion forming the anal membrane. Deviations of the uro-rectal septum can lead to a variety of anomalies, including persistent cloaca, recto-vesical fistula and recto-urethral fistula.
Meanwhile, there are also changes occurring externally. As the urogenital septum is migrating caudally, the intermediate masses are also proliferating. These mesodermal masses form a genital tubercle at the ventral tip of the cloacal membrane. Labioscrotal swellings and urogenital folds develop shortly thereafter on each side of the cloacal membrane.
When the urogenital septum reaches the cloacal membrane, dividing it into its two components parts, the labioscrotal folds fuse yielding a perineum which separates the anal membrane from the urogenital membrane. The embryo is still in its bipotential phase at this point and has the potential to develop either male or female external genitalia.
