· Frog is unisexual.
· Well-marked sexual dimorphism is seen.
· Males can be differentiated from females by:
  1. More muscular upper limbs.
  2. Vocal sacs at the ventral surface of the head.
  3. Nuptial pads at the base of the index finger of the forelimb.
· Fertilization is external.
· Larval stage is present during the development.

Male Reproductive Organs

1. Testes
· Paired, oval bodies lying ventral to kidneys to which they are suspended by a double fold of peritoneum called mesorchium.
· Histologically, testis consists of a large number of Seminiferous tubules → lined by germinal epithelium that gives rise to sperms (spermatogenesis).
· Between the tubules: Interstitial cells or Leydig's cells → secret Testosterone which maintains secondary sexual characters.
· Fat bodies are attached to the anterior end which is used in hibernation and aestivation for the energy of a frog.

2. Vasa Efferentia
· Arise from testes 10-12 in number on each side.
· Open into the Bidder’s canal.
· Note: Bidder's canal is found in the Kidney and in relation to Testis.

The male reproductive system of frog

3. Urinogenital duct (Ureter)
· Begins as a longitudinal tubule on the outer aspect of the kidney.
· Bidder’s canal is connected to the ureter by means of a series of transverse collecting tubules.
· Ureter emerges from the posterior fourth of the outer surface of the kidney.
· Slightly enlarged after emerging from the kidney to form a seminal vesicle.
· Opens into the dorsal surface of the cloaca.

Female Reproductive Organs

1. Ovaries
· Paired lobulated sac-like structures lying ventral to kidneys and suspended by a peritoneal fold called mesovarium.
· Fat bodies are attached at the anterior end.
· Histologically, the ovary consists of three layers- outer theca external, middle germinal epithelium and inner theca interna.
· The surface of the ovary is lined by germinal epithelium which gives rise to the ovarian follicles each with an ovum and follicular epithelium.
· Follicular cells surrounds, protects and provide nutrition to developing ovum or oocyte.
· During the breeding season, the wall of the ovary ruptures and ova are released into the coelomic cavity by ovulation process.

2. Ovum
· Spherical in shape, 2 mm in diameter, bounded by a vitelline membrane secreted by the ovum itself.
· Yolk is concentrated to one pole, i.e. Telolecithal.
· Yolk is present in adequate amount, i.e. Mesolecithal.
· Ovary has no internal connection with the kidney.

The female reproductive system of frog

3. Oviducts or Mullerian ducts
· A pair of long, coiled thick walled ducts that consist of:
a) Oviducal funnel: Lies towards the base of lungs and opens into coelom through a ciliated opening called ostium.
b) First part of oviduct: Thin and narrow.
c) Middle part of oviduct: Longest, much coiled and thick-walled.
· Secrets an albuminous substance that coats the eggs as they pass down.
· It absorbs water and swells to form a coherent mass called frog-spawn.
d) Final portion of oviduct: Broad, thin-walled and known as Ovisac / Uterus.
· It collects ova before being laid.
· A mature female can lay 2500 to 3000 ova at a time.

Fertilization and Metamorphosis

· Mating takes place in the breeding/rainy season.
· Due to the absence of genital organs, there is pseudo-copulation or amplexus i.e. sexual embracement.
· Female releases eggs from cloaca while male releases sperms over the eggs.
· The eggs and sperms unite together outside the body so external fertilization takes place.
· Development is by metamorphosis from the egg-tadpole-adult stage.
· The larval stage is called tadpole.
· The tadpole undergoes metamorphosis and develops into an adult frog.
· Metamorphosis is accelerated by Thyroxine or Iodine whereas delayed by Thiourea.

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