Before the anal opening forms the primitive caudal intestine receives from above the stalk of the allantois, while the Wolffian duct, the canal of the embryonic excretory apparatus, also opens into it. The renal bud on the Wolffian duct in [Fig. 28] indicates the beginning development of the permanent kidney (metanephros), and the proximal portion of the allantoic stalk is destined to form by a spindle-shaped enlargement the future urinary bladder ([Fig. 28]). The caudal gut has as yet no anal opening. Ventrad of the tail end of the embryo the ectoderm presents at this time a depression (Fig. 21). The ectoderm lining the bottom of this anal fossa or depression is separated by a little mesoderm tissue from the entodermal lining of the blind pouch of the caudal gut. Ectoderm and entoderm in this region with the intervening mesodermal layer form the cloacal membrane (Fig. 21).
Development of Cloaca.—The entodermal pouch or prolongation sent down from the end-gut to meet the anal invagination enlarges and dilates to form a short wide piece of the intestinal tube into which open on the one hand the urinary and sexual ducts of the genito-urinary system, while it receives on the other the termination of the end-gut proper ([Figs. 28] and [29]).
Fig. 23.—Genito-urinary tract and cloaca of Iguana tuberculata, ♀. (Columbia University Museum, No. 1846.)
This is the permanent condition of the terminal openings of the alimentary and genito-urinary tracts in the lower vertebrates. It is found in certain fishes, in all amphibia, reptiles and birds, and occurs also in one order of mammals, the monotremes. In man and mammals generally the anal orifice is separated from the genito-urinary opening, lying dorsad of the same and provided with special sphincters. Only in the monotremes do the anus and the genito-urinary tract open into a common cloaca surrounded by a sphincter common to the anal and genito-urinary openings (sphincter cloacæ). In birds, reptiles, amphibia and many fishes (especially the Plagiostomata) this cloacal formation is the rule. In many fishes, especially the Teleosts, the anus and the genito-urinary openings are separate, as in mammals, but their position is reversed, the anus being ventral, while the genito-urinary opening is placed dorsally.
Fig. 23 shows the cloaca in a female specimen of Iguana tuberculata. The ventral wall of the cloaca has been divided to the left of the median line and turned over to the right, carrying with it the cloacal opening of the bladder. The termination of the alimentary canal opens into the cloaca from above.
A transverse fold of the mucosa separates this upper compartment of the cloaca (coprodæum) from a lower space (urodæum) which receives in its dorsal wall the openings of the two oviducts and immediately above them—upon two papillæ—the openings of the ureters, while the ventral wall contains the cloacal opening of the bladder.
The right ovary has been removed—to show the abdominal opening of the right oviduct—by dividing the mesovarian peritoneal fold.
Fig. 24.—Genito-urinary tract and cloaca of the hen, Gallus bankiva. (Columbia University Museum, No. 1208.)