The frog is a unisexual animal with easy sexual dimorphism. Females are smaller than males of the same age. The abdomen of male frog is small and thin. Vocal cords are present in a male.
The male reproductive system of frog consists of following organs;
Testes: It is one paired, yellowish colour, oval shaped which is encircled by peritoneum (mesorchium ) and are attached to the dorsal wall of the kidney. Each testis consists coiled structures called seminiferous tubules that are connected each other by connective tissues. It consists of interstitial cells whose secretion enable to show secondary sexual characters. Each tubule is externally covered by membrane prepared and internally by germinal epithelium. The epithelial lining of seminiferous tubules consists of germinal cells, which produce spermatozoa.
Sperm is formed of head, neck and tail. The head is thick nd enlarged structure with acrosomes in its top which contains the same sized nucleus. Neck joins head and tail parts which consist of centrosome and mitochondria. The tail is a thread like structure and helps in wriggling movements of sperm.
Vasa efferentia: It is a group of 10- 12 ducts that arise from testis and opens into Bidder's canal. Internally it is covered by epithelium and externally by connective tissues. At last, it opens collectively into a network of ducts known as rete testis.
Seminal vesicle: Each urinogenital duct expands to form seminal vesicle where the sperms are stored until they are ejected out during copulation. It contains fructose which nourishes the sperm and its internal structure is same as of vas differential.
Urino-genital ducts: Also called ureter or collecting duct runs below the kidneys. The sperms pass out through this duct along with urine.
Spermatogenesis is defined as the formation of sperm in the genital epithelium. It consists of four stages,
Multiplication phase: Genital epithelium cell is divided by mitotic division into numerous cells called spermatogonia. The chromosomes number remains the same as 24.
Growth phase: The spermatogonia enlarge in their size by absorbing nutrients and are called as primary spermatocytes. The chromosomes number remains the same.
Maturation phase: Here, primary spermatocyte forms two cell by meiotic division. Here the chromosome number reduces to half and comes to 4. These cells are further divided and each primary spermatocyte is converted into four secondary spermatocytes.
Metamorphosis: This stage is the stage of formation of sperm, mitochondria and centriole forms head, Golgi complex gives rise to acrosome and centriole forms tail and its sheath is formed from mitochondria.
It consists of following parts
Ovary: Paired yellowish black coloured structure covered by a membrane of peritoneum known as mesovarium. This is full of eggs during breeding season.
Ovaries are small structures consisting three layers, outer theca external, middle germinal epithelium and inner theca internal which consist blood cells, muscle cells, nerves cells and germinal epithelium. From germinal epithelium, single epithelium enlarges and forms the egg. The ova terminates from the abdominal cavity and comes out which is known as ovulation.
Oviduct: It is a long coiled duct with the thick wall that is not connected with kidneys. It is divided into three parts.
Oviduct funnel: wide opened structure, located in the anterior part of body cavity near lungs and oesophagus. The ciliated wide opening is known as the ostium.
Middle region: coiled part with thin body wall in some area and thick in the remaining area. It consists some glands which secrete albumen that hardens ova.
Uterus: It is the middle part near the cloaca which is wide, thin-walled structure and forms uterus.
After the ovulation, the cilia of the oviducal funnel draws the coelomic fluid with ova which moves into the middle region with the help of cilia. Here, ova is encircled by albumen and later it comes into the uterus.
Eggs are formed from the germinal epithelium. They become mature after passing through the following stages,
Multiplication phase: Genital epithelium divides into numerous cells called oogonia by mitotic division.
Growth phase: After absorbing nutrients, these cells become large which are called primary oocytes. Here, chromosome number is 22.
Maturation phase: In the first meiotic cell division each cell is divided into a large secondary oocyte and smaller known as first polar bodies. The first polar bodies disintegrate. The second meiotic division produces the ovum proper and the second polar body. Two vitelline membranes bound the egg, which is encircled by albumen. A nucleus is seen in the middle part that is surrounded by protoplasm. Each egg has two colours, white is due to yolk that lies downward and black part is of less weight that lies on the surface of the water. These are also called vegetal and animal pole respectively.