The right ovary has been removed--to show the abdominal opening of the right oviduct--by dividing the mesovarian peritoneal fold.
[Illustration: FIG. 24.--Genito-urinary tract and cloaca of the hen, _Gallus bankiva_. (Columbia University Museum, No. 1208.)]
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_.
[Illustration: FIG. 25.--Genito-urinary tract and cloaca of _Platypus anatinus_, duck-billed platypus. (Columbia University Museum, No. 1802.)]
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.
The semen enters the penis at its root through a narrow opening situated close to the junction of genito-urinary sinus and cloaca.
For a short period, therefore, the human embryo and the embryos of the higher mammalia present conditions which correspond to the permanent structure of the parts in these lower vertebrates. In human embryos of 11.5 mm. cervico-coccygeal measure (32-33 days) (Fig. 28), the cloaca appears as a short sac continuous dorsad with the intestine, ventrad with the rudiment of the urinary bladder. The larger portion of the caudal gut (postanal gut) has disappeared, having been reduced to a thin epithelial strand which gradually becomes entirely absorbed. Only the proximal portion of the end-gut is used for the development of the cloaca, which, however, at first has no external opening (Fig. 28).