Q.Describe the reproductive system of the frog, including the reproductive organs of the male and the female, the seasonal breeding cycle, amplexus, and the process of external fertilisation.

Step 1 — Male reproductive organs. A pair of small, yellowish testes lie attached to the front border of the kidneys by the mesorchium. Fine vasa efferentia carry sperm from the testes into the adjoining kidney, from where the sperm pass through the kidney's own ducts and are released via a shared urinogenital duct opening into the cloaca; the male frog has no separate penis, since sperm transfer does not occur internally.

Step 2 — Female reproductive organs. A pair of large, sac-like ovaries, which swell enormously with pigmented eggs as the breeding season approaches, release their mature eggs by rupture directly into the body cavity; ciliary currents sweep the eggs into a pair of coiled oviducts, which coat each egg in protective jelly as it passes down, before opening into the cloaca.

Step 3 — Seasonal breeding cycle. Frogs are markedly seasonal breeders, with breeding activity in West Bengal, as across most of India, tightly synchronised to the onset of the monsoon rains, when ponds and shallow water bodies fill and become suitable breeding and larval-development sites.

Step 4 — Amplexus. As the rains begin, males gather at breeding sites and call loudly, using inflated vocal sacs, to attract females. When a female approaches, the male climbs onto her back and clasps her firmly using roughened nuptial pads that develop on his first digit specifically for the breeding season — this mating embrace is called amplexus and may last for hours. …

Describe the male and female reproductive organs and their ducts in turn, then describe the breeding cycle chronologically — season trigger, …

• Assuming the frog has a penis or other internal sperm-transfer organ, when fertilisation is external and no such structure exists. …