[Illustration: FIG. 28.--Reconstruction of caudal end of human embryo of 11.5 mm. (four and a half weeks), showing pelvic structures. × 40. (After Keibel.)]
=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).
[Illustration: FIG. 29.--Reconstruction of caudal end of human embryo of 14 mm. (five weeks). × 20 (After Keibel.)]
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.
[Illustration: FIG. 23.--Genito-urinary tract and cloaca of _Iguana tuberculata_, female. (Columbia University Museum, No. 1846.)]
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.