After closure of the neuro-enteric canal and obliteration of the post-anal gut the alimentary tube ends, during a short period, both cephalad and caudad in a blind pouch. Very soon, however, the ectoderm becomes invaginated at both extremities and finally perforates into the lumen of the intestine, thus establishing the oral and anal communications with the exterior. The anal ectodermal invagination (proctodaeum) (Fig. 21), is smaller than the oral (stomadaeum) (Fig. 27), but the intestinal tube forms an extensive pouch in the anal region which descends to meet the ectodermal invagination of the proctodaeum. The details of the embryonic processes leading to the final establishment of the adult condition are of great interest on account of the pathological importance of abnormal or arrested development in these parts. Failure of the caudal intestinal pouch to establish a communication with the anal invagination, or failure of development in either anal invagination or intestinal pouch, leads to the condition known as atresia ani or imperforate anus, of which there are several varieties. [Illustration: FIG. 27.--Median section through head of embryo rabbit of 6 mm. (Mihulkovics.)] 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). [Illustration: FIG. 28.--Reconstruction of caudal end of human embryo of 11.5 mm. (four and a half weeks), showing pelvic structures. x 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). x 20 (After Keibel.)]