The frog cloaca is a short simple tube receiving at its inner end the genital and urinary ducts, the rectum, and the allantoic bladder. The female cloaca diners from the male only in the addition of the Mullerian ducts. The ducts open on a ridge of vacuolated tissue marking the boundary of cloaca and rectum. It functions as part of a recto-cloacal valve and brings the urinary openings close to the opening of the bladder. The lining epithelium of the cloaca is derived half from ectoderm and half from endoderm; the former in its stratification and glands resembles the skin, the latter is composed of tall glandular cells with some resemblance to those of the rectum. The general structure of the organ is similar throughout; the epithelium is backed by a layer of loose connective tissue containing lymph spaces; this in turn is enclosed by a layer of first circular and then longitudinal muscles embedded in dense connective tissue, to the outside of which are attached the extrinsic muscles of the cloaca.

A sequence of changes occurs in the endodermal epithelium and the connective tissue beneath it which can be correlated with stages in the sexual cycle. Following the breeding season there is a collapse of the individual epithelial cells and of the folds, the latter apparently due to a reduction in the lymph spaces of the loose connective tissue; during the late summer and autumn the cells and folds are gradually built up to an intermediate condition which persists over the winter, reaching a final enlargement as the breeding season approaches. It is suggested that the function of this tissue is to flush out and lubricate the cloaca, particularly for the passage of eggs and sperm. Injection of growth-stimulating extract of the anterior pituitary brings about an acceleration of the building-up process, slight in June but marked in October, and injection of gonad-stimulating extract appears to produce a tendency towards the post-breeding condition, so it is suggested that this part of the cloaca is functionally part of the genital duct system, which is, according to Spaul and Gladwell, under pituitary control. These and other injections produce in the ectodermal part of the cloaca only the effects that they produce upon the skin itself.