Oviduct - Fish and Amphibians

Fish and Amphibians

In amphibians and lungfishes, the oviduct is a simple ciliated tube, lined with mucus-secreting glands that produce the jelly that surrounds the ovum. In all other vertebrates, there is normally some degree of specialisation of the tube, depending on the type of eggs produced.

In cartilaginous fishes, the mid portion of the tube develops as a shell gland. The first portion of this gland secretes the egg white, while the lower portion secretes a hard, horny, capsule to protect the developing egg. Below the shell gland is the ovisac, a distended region in which eggs are stored prior to laying. In ovoviviparous species, the egg remains within the ovisac until it hatches. Some cartilaginous fishes, however, are truly viviparous, giving birth to live young, and producing no egg shell. In these forms the ovisac nurtures the developing embryo, often with the aid of vascular outgrowths similar to, but much simpler than, the mammalian placenta.

The most primitive ray-finned fishes retain the simple structure also found in lungfishes, but in teleosts, folds of peritoneum enclose the ovary and upper part of the tube, fusing them into a single structure. The ovary itself is hollow, with eggs being shed into the central cavity, and thence passing directly into the oviduct. The enclosed nature of the female reproductive system in these fishes makes it impossible for eggs to escape into the general body cavity; a necessary development given that thousands or even millions of eggs may be released in a single spawning.