The development of the mesonephros does not take place till long after that of the pronephros.
In both forms the ducts unite behind with the cloaca, and a pronephros of the Teleostean type appears to be developed.
The great distinction of the pronephros is that it, and it alone, forms the segmental duct.
Eventually however the formation of the mesonephros commences, and is followed by the atrophy of the pronephros.
The knob forms the only structure which can be regarded as a rudiment of the pronephros.
The pronephros has only been observed in Birds, and is very possibly not developed in other Amniota.
For a long time the pronephros and its duct form the only excretory organs of larval Amphibia.
The mode of development of the pronephros itself is apparently partly dependent on that of its duct.
For a considerable period the pronephros constitutes the sole functional part of the excretory system.
In Amphibia (Salamandra) the permanent opening of the Mllerian duct is formed independently, some way behind the pronephros.
pronephros pro·neph·ros (prō-něf'rəs, -rŏs')
n. pl. pro·neph·roi (-roi) or pro·neph·ra (-rə)
A kidneylike organ, being either part of the most anterior pair of three pairs of organs in a vertebrate embryo, usually disappearing early in embryonic development.