The Female Reproductive Organs
These consist of the ovary and oviduct. Generally only the left organs are functional in birds, and the right set regresses after development. The avian oviduct represents the entire genital tract and extends from the ovary to the cloaca.
The gonad and tubular tracts of both male and female fill much of the body cavity while productive but, when inactive outside of the breeding season, regress to such an extent that they may be difficult to locate.The Ovary
In the first 5 months after hatching, the ovary gradually develops from a small irregular structure with a finely granular surface to one in which individual follicles can be distinguished. These then rapidly increase in number and size until some are several centimeters in diameter (the size of an egg yolk (Figs. 37.33 and 37.34 and see Fig. 37.24/7). The mature ovary resembles a truss of grapes, of various sizes, that is broadly attached to the cranial division of the left kidney. It contains several thousand follicles—far more than the number of eggs (about 1500) laid by even the most productive hen. The larger follicles are pendulous and make contact with the stomach, spleen, and intestines. Each consists of a large, yolk-filled oocyte surrounded by a highly vascular follicular wall. Shortly before ovulation, a devascularized white band (stigma) appears opposite the stalk, indicating where the wall will rupture at ovulation (Fig. 37.35/2 and see Fig. 37.34). The empty follicle (calix) regresses after ovulation and disappears in a few days. No corpus luteum is required because there is no embryo to maintain within the bird's body.
FIG. 37.32 Cloaca of a drake with protruded phallus whose tip has been cut off, dorsal view. 1, Colon; 2, coprodeum; 2', Coprourodeal fold; 3, urodeum; 4, ureteric orifice; 5, papilla of deferent duct; 6, proctodeum; 6', proctodeal glands; 7, lip of vent; 8, spiral groove of phallus; 8', beginning of spiral groove.
The Oviduct
The oviduct conducts the fertilized ovum to the cloaca, adds substantial amounts of nutrients (including the albumen), and encloses it with membranes and a shell to protect the developing embryo. It conveys spermatozoa to the ovum for immediate fertilization and may store them for a time for future use. In the chicken, one insemination is sufficient to fertilize the ova released during the following 10 days or so.
FIG. 37.33 Ventral view of reproductive organs of a hen. 1, Ovary with follicles in different stages of development; 2, oviduct; 3, uterus; 4, rectum; 5, cloaca.
The oviduct (Fig. 37.35/3-7 and see Figs. 37.33 and 37.34) may be divided into infundibulum, magnum, isthmus, uterus, and vagina according to the function of its parts; the uterus and vagina are, of course, not analogous to the like-named organs of mammals. The oviduct occupies the left dorsal part of the body cavity, where it is related to the kidney, intestines, and gizzard. It is a massive coil, approximately 60 cm long (i.e., about twice the body length) when fully functional but much smaller in juveniles and during the nonlaying period. It is suspended from the roof of the body cavity by a peritoneal fold (mesoviductus), and some coils are connected by a continuation that forms the prominent muscular ventral ligament (Fig. 37.35/12). The wall of the oviduct consists of the usual layers: serosa, tunica muscularis (consisting of outer spiral and inner circular layers), a scanty submucosa, and a tunica mucosa containing many glands.
The cranial end is formed by the 7-cm-long infundibulum (Fig. 37.35/3), consisting of fluted and tubular parts. The thin-walled fluted part is stretched to form a slit (infundibular ostium) several centimeters long. Its lateral end is attached to the body wall near the last rib. The ostium is positioned by the left abdominal air sac in such a way that it can grasp newly released oocytes.
The oocyte passes through the infundibulum in about 15 minutes. Fertilization must take place before the infundibular glands provide the chalaziferous layer, the thin coat of dense albumen directly around the yolk. The chalazae—the coiled strands that suspend the yolk and allow it to rotate so that the germinal disk remains uppermost, although part of this layer—develop farther along the genital tract (Fig. 37.36/3'). Some species have an infundibular sperm host gland in which sperm may be stored.
FIG. 37.34 Isolated female reproductive organs. 1, Ovary with follicles in different stages of development; 2, oviduct; 3, uterus; 4, rectum; 5, cloaca.
The highly coiled magnum (Figs. 37.33, 37.34, and 37.35/4) measures about 30 cm and is the longest segment of the duct. Its walls carry massive mucosal folds and are thickened by the glands that contribute about half the total albumen to the egg. Calcium, sodium, and magnesium are also added here. The mucosal folds are lower and the secretion more mucous in the distal end of the magnum. The egg takes about 3 hours to pass through this part.
The isthmus (Fig. 37.35/5), about 8 cm long, is demarcated from the magnum by a narrow, translucent glandular zone. The isthmus, thinner and with lower mucosal folds than the magnum, secretes more albumen and also a material that rapidly congeals to form the two homogeneous membranes found between the albumen and the shell. The egg takes upward of 1 hour to traverse the isthmus. The isthmus is lacking in psittacines.
The uterus (shell gland; Fig. 37.35/6) that follows the isthmus is about 8 cm long; it is a thinner walled and slightly enlarged chamber. Its mucosa bears many low folds and ridges that flatten themselves against the egg, which remains here for about 20 hours. Passing through the permeable membranes, some watery albumen is added to the egg. This secretion is then followed by the deposition of the shell and shell pigments and an outer glazing or cuticle.
FIG. 37.35 Ventral view of the reproductive organs of a laying hen (semi-schematic). L, Left; R, right; 1, ovary; 2, stigma on mature follicle; 3, infundibulum; 4, magnum; 5, isthmus; 6, uterus containing egg; 7, vagina; 8, rectum; 9, cloaca; 10, vent; 11, vestigial right oviduct; 12, free border of ventral ligament of oviduct; 13, outline of right kidney; 14, right ureter.
FIG. 37.36 A semi-schematic section of a fertilized egg. 1, Yolk; 1', yolk membrane; 2, latebra; 2', germinal disk; 3, chalaziferous layer; 3', chalaza; 4 and 4', thin and dense albumen, respectively; 5, internal and external shell membranes; 5', air cell; 6, shell; 7, cuticle.
The final part, the vagina (Fig. 37.35/7), is a muscular, S-shaped tube through which the completed egg passes in seconds when it is laid. Its junction with the uterus is marked by a sphincter. Glandular crypts in the region of the sphincter have been found to store sperm. The vagina ends at a slitlike opening in the lateral wall of the urodeum. When the egg is laid (blunt end first), the vaginal opening protrudes through the vent, which minimizes contamination by the feces. Sperm host glands, where sperm can be stored for many months, may also be found at the uterovaginal junction. A remnant of the right oviduct (Fig. 37.35/11) is found on the right side of the cloaca; it may become cystic and enlarged.