REPRODUCTIVE SYSTEM
Amphibians have paired ovaries or testes. In the male, sperm travels from the testes, through the Wolffian duct, to the cloaca. In the female, follicles develop on the ovaries and, following rupture, the ova are released into the celom.
Cilia in the celom direct the ova into the infundibulum and then into the oviduct (Stebbins & Cohen 1995; Wright 2001c). A notable anatomic feature of bufonid frogs is a Bidder's organ. This structure is a remnant of ovarian tissue found on the testes, and immature ova are evident histologically. This should not be interpreted as hermaphroditism (Green 2001; Stebbins & Cohen 1995; Wright 2001c).Sexual dimorphism is present in some amphibians, but absent in many. Of the species commonly encountered in practice the following guidelines may be observed. In the bullfrog (Rana catesbeiana), males have larger tympanic membranes than females; male White's treefrogs (Pelodryas caerulea) develop nuptial pads during breeding season whereas females do not; the male dyeing poison frog (Dendrobates tinctorius) has large triangular toes, the female has smaller, more rounded toe tips, and the mature male red-eyed treefrog (Agalychnis callidryas) is smaller than the female (Stebbins & Cohen 1995; Wright 2001c).
Gonad activity and size fluctuate with reproductive state. Depending on the species, breeding season may be influenced by temperature, rainfall, or changes in day length. Vocalization of other individuals may also contribute to breeding synchrony among anurans (Stebbins & Cohen 1995).
Caecilians copulate and fertilize internally. The everted cloaca of the male forms the phallodeum, and deposits sperm into the female's cloaca (Stebbins & Cohen 1995; Wright 2001c). Approximately 75% of the caecilians are viviparous (Mitchell et al. 1988) and the oviductal lining may be consumed by the developing young as a food source (Wright 2001c).
A great majority of the salamanders are internal fertilizers. The males lack an intromittent organ, and instead deposit sperm packets, or spermatophores, on the substrate. The female picks up these packets through the cloacal opening and they are stored in the cloaca until egg laying. The exceptions are the Asiatic land salamanders (Hynobiidae) and the giant salamanders (Cryptobranchi- dae), which release sperm onto the egg mass once it is deposited outside the body (Stebbins & Cohen 1995; Wright 2001c).
The number and size of the ova produced vary greatly among species. The ova are typically surrounded by a translucent, gelatinous envelope and deposited in clusters in fresh water or moist terrestrial habitats. Melanic pigmentation of the ova is thought to protect against UV radiation and concentrate heat to warm them (Stebbins & Cohen 1995). Incubation duration varies from hours (24 hours for the black toad, Atelopus spp.) to several months. At the time of hatching, glands on the snouts of the larvae produce enzymes that dissolve the egg capsules. The duration of the larval stage depends on species and temperature (Stebbins & Cohen 1995; Wright 2001c).