In the normal rats on gestational days 15, URS fuses with cloacal membrane, and on gestational days 16 anal membrane ruptures and rectum communicate with the anus.

• 鼠胚胎 15d时尿直肠隔与泄殖腔膜融合。 胚胎 16d肛膜完全破裂 ,直肠与外界相通。