Category : 12th Class
(1) Breeding : Frog breeds in the rainy season, June to September. Male frogs produce crocking sound (mating call) by their vocal sacs.
(2) Ovulation : The eggs in the stage of secondary oocytes are released into the body cavity by rupture of ovary during ovulation.
(3) Spawning : Spawning is the act of laying of eggs by the female frog stimulated by the male during amplexus.
Spawn is a cluster or mass of eggs laid by a female. a spawn of Rana tigrina contains about \[3000-4000\] eggs. Gelatin protects the egg from predators and also acts as an insulator keeping the egg warm.
(4) Fertilization : Fertilization in frog is external taking place in water. When a sperm enters into the egg of frog, second meiotic division occurs. Sperm entrance point marks the anterior side of the future embryo. Gray crescent marks the dorsal side of the future embryo. A gray crescent appears in the equitorial zone geometrically opposite to the sperm entrance. The bilateral organization is established at the time of sperm penetration. The region where sperm enter the egg cell is called ‘reception cone’.
Structure of egg : Frog’s egg is mesolecithal (based on distribution of yolk). Upper black of darkly pigmented part is animal hemisphere. Lower unpigmented or white part is vegetal hemisphere.
(5) Cleavage : First cleavage of frog is meridional passing through median longitudinal axis, holoblastic and equal. Gray crescent is present only in two blastomeres of future dorsal side.
All divisions from third cleavage are unequal holoblastic. Holoblastic equal cleavage in frog ends after second cleavage division. Third cleavage plane is horizontal, but above the level of equator (latitudinal).
Smaller cells are micromeres and larger ones are called macromeres. The micromeres contain no yolk and macromeres contain large amount of yolk.
Fourth cleavage involves two synchronous meridional divisions resulting in the formation of 16 blastomeres.
Fifth cleavage involves two simultaneous latitudinal divisions resulting in the formation of 32 blastomeres.
After sixth or seventh cleavage division, the embryo looks like a mulberry-shaped ball of cells. This called morula stage.
(6) Morula : It is a solid ball of cells. A cavity called blastocoel appears towards animal hemisphere. The blastula of frog is called coeloblastula.
In frog, there is a regulative development. The cleavage is indeterminate. If one of the two blastomeres of frog is damaged, the development will be normal.
(7) Gastrulation : Blastula is a hollow ball of cells. By the end of gastrulation, it is converted into a three-layered embryo made of ectoderm, mesoderm and endoderm often enclosing an archenteron.
Gastrulation includes three kinds of morphogenetic movements of cells namely - Epiboly of ectoderm, Invagination of endoderm, Involution of chordamesoderm.
Invagination of prospective endoderm cells occurs below equator, exactly below the midpoint of gray crescent of blastula. It results in the formation of a slit later giving rise to blastopore.
Posterior side of future tadpole is represented by the side of frog’s embryo bearing the yolk plug.
(8) Organizer : The dorsal lip of blastopore in the amphibian gastrula is called primary organizer. The theory of organizer (inductor) in amphibian was introduced by Spemann in 1938. He was awarded Nobel prize for this work.
(9) Neurulation : Neurulation takes place after gastrulation. During this stage a neural tube is formed.
The embryo lengthens along its anterior-posterior axis, neural plate (ectodermal) become thickened and raised above the general level as ridges called neural folds. Neural folds meet and fuse at the mid dorsal line. Neurulation includes the formation of neural tube, notochord and gut.
Formation of notochord is known as notogenesis.
(i) Post neurular development : The development takes place inside egg membrane upto tail bud larval stage.
Hatching occurs in 6th day of embryonic life. During hatching, the young frog is called tadpole larva. Newly hatched tadpole larva remain attached to aquatic plants by its oral sucker.
After 24 hours of hatching, mouth and anus are perforated. The larval body is elongated forming head, trunk and tail.
(ii) External gill stage of tadpole
(a) Just above one day after hatching, the external gill stage starts.
(b) Eyes become fully developed and functional.
(c) Horny jaws with teeth appear along the rim of mouth.
(d) Tail elongates and becomes a powerful swimming organ.
(e) Pronephric kidneys become fully developed. Frog’s tadpoles are ammonotelic. Nitrogenous waste matter excreted by frog’s tadpole is ammonia.
(f) Tadpole is herbivorous (phytophagus), feeds on aquatic plants.
(g) Tadpole has a long coiled intestine because digestion takes place relatively long time.
(iii) Internal gill stage of tadpole
(a) Tadpoles grow older, the hind limb buds and internal gills develop.
(b) External gills are replaced by four pairs of internal gills covered with a fold of skin called operculum.
(c) Operculum encloses a chamber, opercular chamber, opens to exterior by spiracle.
(d) Spiracle is present only on the left side of the tadpole.
(e) During respiration, the water currents enter the mouth, bath the gills of pharynx and exit through the spiracle.
(f) Oral sucker disappear and lateral line receptor develop. This serves to perceive stimuli of movements, currents and vibrations of water.
(g) A tadpole of frog resembles a fish in many features except that the tadpole does not possess paired fins and scales on the body.
(i) Metamorphosis includes morphological, anatomical, physiological and behavioural changes.
(ii) Two or three weeks after breathing with gills, the tadpole larva undergoes drastic changes called metamorphosis.
(iii) Two types of changes during metamorphosis are regressive and progressive.
(iv) Some of the regressive morphological changes are :
(v) Some of the progressive morphological changes are :
(vi) Formation of fore limbs breaking through operculum.
(vii) Replacement of pronephros with mesonephric kidney.
(viii) The tail is shortened by reabsorption with the help of lysosomal enzyme cathepsin. This process is also known as autolysis.
Hormonal control of metamorphosis
(1) Hormonal control of metamorphosis in amphibian was discovered by Gudernatsch (1912).
(2) Metamosphosis occurs only when adequate amount of thyroxine is secreted by thyroid of tadpole.
(3) Thiourea is antithyroid drug, it inhibits metamorphosis of frog.
(4) Neoteny refers to the retention of a larval or embryonic trait in the adult body, e.g., Cartilaginous skeleton in adult chondrichthyes and larval gills in some adult salamanders.
(5) Paedogenesis or paedomorphosis refers to development of gonads and production of young ones by larval or pre adult animal e.g., liverfluke and ambystoma.
(6) Deficiency of iodine in the soil results in the failure of metamorphosis in ambystoma.
(7) Axolotal is the larva of ambystoma, it shows paedogenesis.
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