Fig. 24—taken from a preparation of the hen—shows the typical arrangement of the female genito-urinary tract and cloaca in the birds.

The terminal portion of the alimentary canal, in entering the cloaca, forms an expanded upper cloacal compartment for the accumulation of the excreta, called the coprodæum.

It is separated by a prominent mucous fold from the central compartment, or urodæum which receives the terminations of the two ureters and of the single (left) oviduct. A second fold forms the distal limit of the urodæum and separates it from the lowest cloacal compartment, the proctodæum.

Fig. 25.—Genito-urinary tract and cloaca of Platypus anatinus,duck-billed platypus. (Columbia University Museum, No. 1802.)

 

Fig. 26.—Neuro-enteric canal in section of human embryo of 2 mm. (After Spee.)

Fig. 27.—Median section through head of embryo rabbit of 6 mm. (Mihulkovics.)

Fig. 28.—Reconstruction of caudal end of human embryo of 11.5 mm. (four and a half weeks), showing pelvic structures. × 40. (After Keibel.)

Fig. 29.—Reconstruction of caudal end of human embryo of 14 mm. (five weeks). × 20 (After Keibel.)
Fig. 30.—Human female fœtus, 3.4 cm. long, vertex-coccygeal measure. The external perineal folds separate the anal invagination from the uro-genital opening. (Kollmann.)

Fig. 25 shows the male genito-urinary tract and the cloaca in the monotreme, Platypus anatinus. The cloaca is a spacious sac formed by the confluence of the rectum and the genito-urinary sinus.

The penis, consisting of two large cavernous bodies, is contained in a fibrous sac which arises from the junction of the genito-urinary sinus and the cloaca, and is continued into the ventral wall of the cloaca near its termination by an opening through which the penis can pass into the cloaca and beyond the external cloacal aperture.