The cloacal membrane is an area where the endoderm and ectoderm remain in contact at the caudal end of the embryonic disc, at this stage it is more a cord of cells than a membrane. See Carnegie Embryo No. 7801 Florian J., (1933). However by definition the membrane does not contain an intervening layer of mesoderm cells although photographs of sections are not provided in order to verify this.
This is the first stage in which a cloacal membrane appears as a membrane rather than a cell cord connecting endoderm to ectoderm. It can be observed in Embryo Bi24. Florian J., (1933)
The cloacal membrane rotates and a mesenchymal condensation appears in the midline, the genital tubercle, which is indicated in a reconstruction of Carnegie Embryo No. 792 O'Rahilly R., Müller F., (1985) Fig 10 and also see Gasser R.F.,(1975) p111
According to Spaulding M.H., (1921) a shallow depression extends the length of the tubercle, known as the urethral groove and the cloacal membrane sits at the base of this groove. Although the internal separation of the cloaca by the urorectal septum is not yet complete, a transverse bar of mesenchyme apparently spans the urethral groove forming an anal pit at the basal end of the tubercle.
The urorectal septum is well marked, separating the cloaca, the cavity internal to the cloacal membrane, into the future rectum and urogenital sinus. Gasser R.F.,(1975) p110.
The urorectal septum fuses with the cloacal membrane late in this stage, dividing the cloaca into the rectum and urogenital sinus, the cloacal membrane into the anal membrane and urogenital membrane, and creating a peroneal area between the two.
According to O'Rahilly R., Muecke E.C., (1972) the urogenital membrane is ready to rupture
The urogenital membrane ruptures from urinary pressure during this and the previous stage. O'Rahilly R., Müller F., (1987) p244
According to O'Rahilly R., (1978) the anal membrane may begin to break down at this stage