Oviduct

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The oviduct is the tube that carries eggs from the ovary. In humans it’s called the fallopian tube. The egg travels along the oviduct and may be fertilized by sperm to form a zygote, or it may degenerate inside the body. Most animals have two oviducts, but in birds and some other animals one side may not develop, leaving only one working oviduct.

In many animals the oviduct does not touch the ovary directly. The front end ends in a funnel-shaped infundibulum that catches the egg as it is released.

Jawless fishes are the only female vertebrates without an oviduct; they release eggs into the body cavity and through a genital pore.

In amphibians and lungfishes, the oviduct is a simple, ciliated tube lined with mucus glands that make the jelly around the egg.

In other vertebrates the oviduct becomes more specialized depending on the type of egg. In cartilaginous fishes, the middle part develops into a shell gland: the first part makes egg white and the lower part makes a protective shell. Behind the shell gland is the ovisac, where eggs are stored before laying. In species that are ovoviviparous, the egg stays in the ovisac until it hatches. Some cartilaginous fishes are truly viviparous and give birth to live young, with no eggshell; the ovisac nourishes the embryo, sometimes with simple placenta-like structures.

Primitive ray-finned fishes have a simple structure similar to that of lungfishes. In teleosts, folds of the peritoneum wrap around the ovary and the upper part of the tube, joining them into one structure. The ovary is hollow, eggs are shed into the central cavity, and then pass directly into the oviduct. This enclosed system helps keep eggs inside the body when thousands or millions are released.

In amniotes—reptiles, birds, and mammals—the egg is surrounded by an amnion, which encouraged further development of the oviduct. In reptiles, birds, and monotremes the main part of the oviduct is a muscular tube that can stretch to move large eggs and is lined with glands that secrete egg white. The lower part, called the uterus, has thicker muscle and glands that secrete the eggshell. In marsupial and placental mammals, the uterus develops further and is lined with endometrium; in many placentals the two uteri fuse into one, while in marsupials they stay separate.

Birds have a highly specialized oviduct with multiple sections that add the components of the egg as it passes along.