Bidder's organ is a spherical, brownish reproductive organ in any male member of the family Bufonidae, or simply, toads. It is also found in very young frogs, however, before reaching adulthood, the organ regresses completely. In toads, the organ is located just in front of the kidney, or, mesonephros. It is a rudimentary ovary, being formed at the cranial tip of the testis during its larval stage. In inactivation, it contains miniature follicles which have the capability to mature. Zoologists have experimented with the physiology of the organ by castrating the male toad of its testes. In doing so, the Bidder's organ takes over the function of the testis, enlarge, and produce viable oocytes, that is, egg cells, and then it produces gonadotropins, which stimulate the growth of the Mullerian ducts to form uterus and oviducts. It therefore becomes a fully sexually functional female, which leads some zoologists to conclude that toads are actually bisexual and paedomorphic.
Its internal anatomy consists of two parts, the central portion, consisting of connective tissue, and rich in blood vessels, and its periphery consists of the cortex, which contains follicles in various stages of development. Molecules of various proteins are present in the outer layer of the follicles, homologous to the zona pellucida of the ovum. The Bidder's Organ's physiology is unique, having no connection with the sexual chaning capabilities of amniotes (i.e., reptiles, avians, and mammals).
Location in a dissected toad
The ideal specimen for dissection is Bufo marinus, or Giant Toad. In the male toad, lift the intestine leftward. In the mesentery of the intestine between the two mesonephros is the Bidder's organ. It is a conspicuous organ, being colored red, and at the cranial poles of the large testes.