Frog Reproductive System

The frog's reproductive system, like its respiratory system, is closely tied to its amphibious life cycle, since fertilisation and the earliest stages of development both take place in water rather than inside the mother's body. In the male frog, a pair of small, yellowish, oval testes lie attached to the anterior (front) border of the kidneys by a thin membrane, the mesorchium. Fine tubules called vasa efferentia carry sperm from the testes directly into the adjoining kidney, from where the sperm travel onward through the kidney's own ducts and are ultimately released to the exterior via a shared urinogenital duct that opens into the cloaca; notably, the male frog has no separate penis or other intromittent organ, since fertilisation, as described below, does not involve internal transfer of sperm.

In the female frog, a pair of large, thin-walled, sac-like ovaries lie in the body cavity; outside the breeding season they are relatively small, but as the breeding season approaches they become enormously distended, packed with thousands of small, pigmented, developing eggs (ova). When mature, the eggs are released by rupture of the ovary wall directly into the body cavity, from where ciliary currents sweep them into the funnel-shaped openings of a pair of long, coiled oviducts; as an egg passes down an oviduct, it is coated in a layer of protective jelly before the oviduct, too, opens into the cloaca, through which the eggs are finally laid. …