 Behind the umbilicus, however, the mesoderm subsequently extends to form the lower part of the abdominal wall and pubic symphysis.By the growth of the surrounding tissues the cloacal membrane comes to lie at the bottom of a depression, which is lined by ectoderm and named the ectodermal cloaca.A birth defect can arise known as a persistent cloaca where the rectum, vagina, and urinary tract fuse to create a common channel or cloaca.