The oviduct in vertebrates is the passageway from an
cartilaginous fishes
, one or the other side fails to develop (together with the corresponding ovary), and only one functional oviduct can be found.
Except in
infundibulum
, which collects eggs as they are released by the ovary into the body cavity.
The only female vertebrates to lack oviducts are the
jawless fishes. In these species, the single fused ovary releases eggs directly into the body cavity. The fish eventually extrudes the eggs through a small genital pore
towards the rear of the body.
Fish and amphibians
In
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 middle portion of the tube develops as a
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. 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.
Amniotes
In amniotes – reptiles, birds, and mammals – the egg is enclosed with an outer layer, or amnion, which has led to further development of the oviduct. In reptiles, birds, and monotremes, the main part of the oviduct is a muscular tube, capable of considerable distension to transport the large eggs that are produced. This part of the oviduct is lined with glands that secrete the components of the egg white. The lower portion of the oviduct, or uterus, has a thicker layer of smooth muscle and contains the glands that secrete the egg shell.
In
placental mammals, the uterus becomes lined by an endometrium,[1]
and is more developed than in egg-laying amniotes. In many placental mammals, the uteri of each side become partially or wholly fused into a single organ, although in marsupials they remain completely separate.
